fix(util): broken URL to auth docs

See https://googlecloudplatform.github.io/google-cloud-node/#//docs/guides/authentication
opens https://googlecloudplatform.github.io/google-cloud-node/#/ and not the auth docs
removing the extra `/` properly routes to https://googlecloudplatform.github.io/google-cloud-node/#/docs/google-cloud/0.45.0/guides/authentication

Relates to #1405

Author: Splaktar

packages/common/src/util.js

@@ -48,7 +48,7 @@ var errorMessage = format([
   'connection.'
 ].join(' '), {
   baseUrl: 'https://googlecloudplatform.github.io/google-cloud-node/#',
-  path: '/docs/guides/authentication'
+  path: 'docs/guides/authentication'
 });
 
 var missingProjectIdError = new Error(errorMessage);

packages/common/test/util.js

@@ -110,7 +110,7 @@ describe('common/util', function() {
       'an authenticated connection.'
     ].join(' '), {
       baseUrl: 'https://googlecloudplatform.github.io/google-cloud-node/#',
-      path: '/docs/guides/authentication'
+      path: 'docs/guides/authentication'
     });
 
     var missingProjectIdError = new Error(errorMessage);