Wording in readme (#731)

README.md

@@ -60,7 +60,7 @@ defmodule MyApp.Guardian do
   def subject_for_token(%{id: id}, _claims) do
     # You can use any value for the subject of your token but
     # it should be useful in retrieving the resource later, see
-    # how it being used on `resource_from_claims/1` function.
+    # how it is being used on `resource_from_claims/1` function.
     # A unique `id` is a good subject, a non-unique email address
     # is a poor subject.
     sub = to_string(id)