Fix link to google-api-php-client repo [skip ci] (GoogleCloudPlatform#526)

Additional minor fixes:

* used Markdown formatting to put links behind text for readability
* used code formatting for placeholder variable names
* fixed heading for consistency
* removed trailing whitespace

Author: mbrukman

compute/helloworld/README.md

@@ -4,15 +4,15 @@
 This is a simple web-based example of calling the Google Compute Engine API
 in PHP.
 
-## Prerequisites:
+## Prerequisites
 Please make sure that all of the following is installed before trying to run
 the sample application.
 
-- PHP 5.2.x or higher [http://www.php.net/]
-- PHP Curl extension [http://www.php.net/manual/en/intro.curl.php]
-- PHP JSON extension [http://php.net/manual/en/book.json.php]
-- The google-api-php-client library checked out locally
-  [https://code.google.com/p/google-api-php-client/] 
+- [PHP 5.2.x or higher](http://www.php.net/)
+- [PHP Curl extension](http://www.php.net/manual/en/intro.curl.php)
+- [PHP JSON extension](http://php.net/manual/en/book.json.php)
+- The [`google-api-php-client`](https://github.com/google/google-api-php-client)
+  library checked out locally
 
 ## Setup Authentication
 NOTE: This README assumes that you have enabled access to the Google Compute
@@ -22,7 +22,7 @@ Engine API via the Google API Console page.
 application.
 - Click on "API Access" in the left column
 - Click the button labeled "Create an OAuth2 client ID..." if you have not
-generated any client IDs, or "Create another client ID..." if you have
+  generated any client IDs, or "Create another client ID..." if you have
 - Give your application a name and click "Next"
 - Select "Web Application" as the "Application type"
 - Click "Create client ID"
@@ -33,12 +33,12 @@ generated any client IDs, or "Create another client ID..." if you have
 
 2) Update app.php with the redirect uri, consumer key, secret, and Project ID
 obtained in step 1.
-- Update 'YOUR_CLIENT_ID' with your oauth2 client id.
-- Update 'YOUR_CLIENT_SECRET' with your oauth2 client secret.
-- Update 'YOUR_REDIRECT_URI' with the fully qualified
-redirect URI.
-- Update 'YOUR_GOOGLE_COMPUTE_ENGINE_PROJECT' with your Project ID from the 
-API Console.
+- Update `YOUR_CLIENT_ID` with your oauth2 client id.
+- Update `YOUR_CLIENT_SECRET` with your oauth2 client secret.
+- Update `YOUR_REDIRECT_URI` with the fully qualified
+  redirect URI.
+- Update `YOUR_GOOGLE_COMPUTE_ENGINE_PROJECT` with your Project ID from the
+  API Console.
 
 ## Running the Sample Application
 3) Load app.php on your web server, and visit the appropriate website in