fixed typos in doc

doc/providers/doctrine.rst

@@ -122,10 +122,10 @@ Using multiple connections::
 
     $app->get('/blog/show/{id}', function ($id) use ($app) {
         $sql = "SELECT * FROM posts WHERE id = ?";
-        $post = $app['dbs']['mysql_read']->fetchAssoc($mysqlQuery, array((int) $id));
+        $post = $app['dbs']['mysql_read']->fetchAssoc($sql, array((int) $id));
 
-        $mysqlUpdate = "UPDATE posts SET value = ? WHERE id = ?";
-        $app['dbs']['mysql_write']->execute($mysqlUpdate, array('newValue', (int) $id));
+        $sql = "UPDATE posts SET value = ? WHERE id = ?";
+        $app['dbs']['mysql_write']->execute($sql, array('newValue', (int) $id));
 
         return  "<h1>{$post['title']}</h1>".
                 "<p>{$post['body']}</p>";