Corrected grammar in contribution guide (#93014)
Corrected the grammar of a sentence in "Implementing Features or Fixing Bugs" section of the contribution guide.
**Before:**
Issues that are labeled first-new-issue, low, or medium priority provide the best entrance point are great places to start.
**After:**
Issues that are labeled first-new-issue, low, or medium priority provide the best entrance point _and_ are great places to start.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93014
Approved by: https://github.com/albanD, https://github.com/kit1980
