doc: update PR title instructions

[rmshaffer]

Issue #, if available:

Description of changes:
Instructions about commit messages should really refer to PR titles, since that's what ends up in the commit log.

Note - after getting approval and merging this, I'll make the same change in the several other Braket repos which contain this content in their CONTRIBUTING.md.

Testing done:

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (fb307e3) to head (0670f96).
Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #994   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          135       135           
  Lines         8948      8948           
  Branches      2010      2010           
=========================================
  Hits          8948      8948           

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[Manvi-Agrawal]

Thanks a ton @rmshaffer for taking up this small change to improve the checklist and the CONTRIBUTING doc.