Add compute engine credentials (#22)

diff --git a/google/auth/credentials.py b/google/auth/credentials.py
index b10e63e..ef28bd7 100644
--- a/google/auth/credentials.py
+++ b/google/auth/credentials.py
@@ -102,7 +102,7 @@
         apply the token to the authentication header.
 
         Args:
-            request (google.auth.transport.Request): the object used to make
+            request (google.auth.transport.Request): The object used to make
                 HTTP requests.
             method (str): The request's HTTP method.
             url (str): The request's URI.