remove stray punctuation from CONTRIBUTING.md
remove stray punctuation from CONTRIBUTING.md

Change-Id: I5ec9f8674f62075c5bc6876c4e764ad80c90e802
GitHub-Last-Rev: 0b1991dfb05730c9ea1782ebf56b6586a2c409b9
GitHub-Pull-Request: golang/gddo#584
Reviewed-on: https://go-review.googlesource.com/c/140378
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gddo,golang/gddo,segmentio/gddo,segmentio/gddo,segmentio/gddo,golang/gddo,golang/gddo,segmentio/gddo