Changed the string Master to master to make it more readable.

docs/best_practices_guide.adoc

@@ -421,7 +421,7 @@ For consistency, role names SHOULD follow the above naming pattern. It is import
 Many times the `technology` portion of the pattern will line up with a package name. It is advised that whenever possible, the package name should be used.
 
 .Examples:
-* The role to configure a Master is called `openshift_master`
+* The role to configure a master is called `openshift_master`
 * The role to configure OpenShift specific yum repositories is called `openshift_repos`
 
 === Filters