usage examples added into readme

Author: sunra

README.md

@@ -24,7 +24,7 @@ Install
 Usage
 -----
 
-'''php
+```php
 use Sunra\PhpSimple\HtmlDomParser;
 
 ...
@@ -35,4 +35,4 @@ $dom = HtmlDomParser::file_get_html( $file_name );
 $elems = $dom->find($elem_name);
 ...
 
-'''
+```