maven.yaml lint checking

Adjust indentation for credentials-binding section to match an example
in the jjb docs.  Not indented enough and value of credentials::file
manifested as null.  Local lint checking progressed farther, lets see
if this helps formal job generation.

Change-Id: Ia48cb3911435e38bdc884cc352815c88e8c69e32
diff --git a/jjb/maven.yaml b/jjb/maven.yaml
index fc44abf..00bb08e 100644
--- a/jjb/maven.yaml
+++ b/jjb/maven.yaml
@@ -120,8 +120,8 @@
     wrappers:
       - credentials-binding:
           - file:
-            credential-id: 'gpg-creds-maven'
-            variable: 'GNUPG'
+              credential-id: 'gpg-creds-maven'
+              variable: 'GNUPG'
 
     builders:
       # Set JDK version