Update CONTRIBUTING.md

Fix broken link due to incorrect markdown

# Before
```
If you'd like to contribute a fix for a bug you've encountered, first read up on
[how to report a bug][reporting-bugs-and-other-issues] and report it so we are
aware of the issue.
```
# After
```
If you'd like to contribute a fix for a bug you've encountered, first read up on
[how to report a bug](#reporting-bugs-and-other-issues) and report it so we are
aware of the issue.
```