Fix spelling mistake in annotate_routes file doc (#645)

lib/annotate/annotate_routes.rb

@@ -10,7 +10,7 @@
 # Yes, it's simple but I'm thick and often need a reminder of what my routes
 # mean.
 #
-# Running this task will replace any exising route comment generated by the
+# Running this task will replace any existing route comment generated by the
 # task. Best to back up your routes file before running:
 #
 # Author: