DOC: Fix spaces and brackets in ValueError message for option_context.
Add a missing space and changed a parenthesis to a bracket in the message of the ValueError raised whenever  the `args` in `option_context(*args)` don't meet requirements.
'Need to invoke asoption_context(pat, val, [(pat, val), ...)).'
->
'Need to invoke as option_context(pat, val, [(pat, val), ...]).'

- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

@pvanmulbregt this looks good, Travis error is unrelated.  Can you push again to make it try again? @jbrockmendel:  "push again" as in "$ git push  -v -f origin"?  That tells me "Everything up-to-date". Or did you mean something else? > That tells me "Everything up-to-date". Or did you mean something else?

Yah, usually I find a or something to fix to get around that. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=h1) Report
> Merging [#22121](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d30c4a0696d5fbdc3c7ce36a9b9b19224a557e09?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `0%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22121/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22121   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files         170      170           
  Lines       50690    50690           
=======================================
  Hits        46672    46672           
  Misses       4018     4018
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <0%> (ø)` | :arrow_up: |
| #single | `42.31% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/22121/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87.17% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22121?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/22121?src=pr&el=footer). Last update [d30c4a0...52a8210](https://codecov.io/gh/pandas-dev/pandas/pull/22121?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/22121?src=pr&el=h1) Report
> Merging [#22121](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d30c4a0696d5fbdc3c7ce36a9b9b19224a557e09?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22121/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22121   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files         170      170           
  Lines       50690    50690           
=======================================
  Hits        46672    46672           
  Misses       4018     4018
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/pull/22121/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `87.17% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22121?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/22121?src=pr&el=footer). Last update [d30c4a0...52a8210](https://codecov.io/gh/pandas-dev/pandas/pull/22121?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @pvanmulbregt 