DOC: contributing: fix asv usage instructions
The current asv usage instructions in contributing.rst may be unclear for PRs with multiple commits.

By default `asv continuous` uses head^ as the commit to compare to, whereas for PRs you usually want to compare to master. Write the instructions so that the comparison point is clear.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13794?src=pr) is 85.33% (diff: 100%)

> Merging [#13794](https://codecov.io/gh/pydata/pandas/pull/13794?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13794   diff @@
==========================================
  Files           141        141          
  Lines         50711      50711          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43275      43275          
  Misses         7436       7436          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3fdcea6...e624f9b](https://codecov.io/gh/pydata/pandas/compare/3fdcea6f89a5a9f9fdb98452343a21e576430bd9...e624f9beba65fc9690dc2f8e501814c83a712130?src=pr)
 @pv Thanks a lot for clarifying this!
 Yes, you can replace HEAD with a branch name, but is not necessary.
 @pv I have actually another questions (was using the asv benchmark suite the last days): is there a way to run the suite for a specific commit? As with `asv run` I think you need to specify a range? 
The situation I wanted this was to compare the benchmarks of two commits (last release vs master), for which I wanted to use `asv compare`, but did not yet run benchmarks for only one of the two commits
 asv run commit^!     (git single-commit range spec)
 @pv Aha, thanks!
