chore: fix typo (#647)

diff --git a/google/auth/compute_engine/credentials.py b/google/auth/compute_engine/credentials.py
index 4ac6c8c..2906310 100644
--- a/google/auth/compute_engine/credentials.py
+++ b/google/auth/compute_engine/credentials.py
@@ -14,8 +14,8 @@
 
 """Google Compute Engine credentials.
 
-This module provides authentication for application running on Google Compute
-Engine using the Compute Engine metadata server.
+This module provides authentication for an application running on Google
+Compute Engine using the Compute Engine metadata server.
 
 """