Fix expected file permissions to match module's current state

Author: William Yardley

spec/acceptance/server_spec.rb

@@ -194,7 +194,7 @@ class { 'mongodb::client': }
         it { is_expected.to be_file }
         it { is_expected.to be_owned_by 'root' }
         it { is_expected.to be_grouped_into 'root' }
-        it { is_expected.to be_mode 644 }
+        it { is_expected.to be_mode 600 }
         it { is_expected.to contain 'db.auth(\'admin\', \'password\')' }
       end