Fix typo :)

lib/rspec/support.rb

@@ -139,7 +139,7 @@ def self.===(exception)
       end
     end
 
-    # The Differ is only needed when a a spec fails with a diffable failure.
+    # The Differ is only needed when a spec fails with a diffable failure.
     # In the more common case of all specs passing or the only failures being
     # non-diffable, we can avoid the extra cost of loading the differ, diff-lcs,
     # pp, etc by avoiding an unnecessary require. Instead, autoload will take