gitignore: remove obsolete reference to .hgignore in comment

Change-Id: Id56bd12269c24fd29c83d17bc61e5189713ffee9
Reviewed-on: https://go-review.googlesource.com/c/sys/+/224337
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/.gitignore b/.gitignore
index 8339fd6..5a9d62e 100644
--- a/.gitignore
+++ b/.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