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 <iant@golang.org>

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 0b3a27f..88b4b30 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -1,7 +1,7 @@
 # Contributing to godoc.org
 
 Contributions to this project are welcome, though please
-[file an issue](https://github.com/golang/gddo/issues/new).
+[file an issue](https://github.com/golang/gddo/issues/new)
 before starting work on anything major.
 
 To get started contributing to this project,