Update rakefile

Minor change to the sentence so that it reads easier.

Author: SteveChurch

Rakefile

@@ -17,7 +17,7 @@ end
 task :verify_private_key_present do
   private_key = File.expand_path('~/.gem/rspec-gem-private_key.pem')
   unless File.exist?(private_key)
-    raise "Your private key is not present. This gem should not be built without that."
+    raise "Your private key is not present. This gem should not be built without it."
   end
 end