gitignore: remove obsolete reference to .hgignore in comment

Change-Id: I24a2e874b55b4a7161bcb7a8919f23af6513ceb2
Reviewed-on: https://go-review.googlesource.com/c/net/+/224317
Reviewed-by: Ian Lance Taylor <iant@golang.org>

.gitignore

@@ -1,2 +1,2 @@
-# Add no patterns to .hgignore except for files generated by the build.
+# Add no patterns to .gitignore except for files generated by the build.
 last-change