Add git prs as an alias for git pull-requests (#64)

## Summary:
Khan/our-lovely-cli#530 added a git
pull-requests command to OLC. This adds an alias for it that's easier to
type.

Issue: none

Test plan:

Run `git prs`. You should see a report of your outstanding pull
requests.

Author: benchristel

Reviewers: benchristel, lillialexis, jaredly

Required Reviewers:

Approved By: lillialexis

Checks:

Pull Request URL: #64

.gitconfig.khan

@@ -27,6 +27,7 @@
   rb = !git review-branch
   pb = !git review-branch       ; short for 'phabricator branch'
   pr = !git pull-request
+  prs = !git pull-requests
 
   # Other useful commands.
   outgoing = "!git fetch >/dev/null 2>&1; git log @{upstream}..HEAD"