Minor touch-ups to GitHub PULL_REQUEST_TEMPLATE
Previously, `PULL_REQUEST_TEMPLATE.md` included spaces before each item in the [GitHub task list](https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments), which caused the tasks to become nested upon GUI reordering.

I made minor changes to other aspects of `PULL_REQUEST_TEMPLATE.md`. I removed the `-u` arg from the `git diff` command since its default. In `contributing.rst`, I switched `-u` to the long form of `--patch`, since it's later juxtaposed to `--name-only`.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=h1) Report
> Merging [#17207](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/556effcba52f4712fed21b269e9782f1a309ea93?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17207/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17207      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.02%     
==========================================
  Files         162      162              
  Lines       49511    49511              
==========================================
- Hits        45070    45061       -9     
- Misses       4441     4450       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.79% <ø> (ø)` | :arrow_up: |
| #single | `40.26% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=footer). Last update [556effc...75b3df6](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=h1) Report
> Merging [#17207](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/556effcba52f4712fed21b269e9782f1a309ea93?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17207/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17207      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.02%     
==========================================
  Files         162      162              
  Lines       49511    49511              
==========================================
- Hits        45070    45061       -9     
- Misses       4441     4450       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.79% <ø> (ø)` | :arrow_up: |
| #single | `40.26% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.71% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=footer). Last update [556effc...75b3df6](https://codecov.io/gh/pandas-dev/pandas/pull/17207?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @dhimmel !