Rename gems key to plugins

Author: mmistakes

docs/_config.yml

@@ -51,7 +51,7 @@ permalink: /:categories/:title/
 timezone: America/New_York # http://en.wikipedia.org/wiki/List_of_tz_database_time_zones
 
 # Plugins
-gems:
+plugins:
   - jekyll-feed
   - jekyll-seo-tag
   - jekyll-sitemap