docs: fix incorrect output

The getPath() returns the path starting with `/`.
But if you set the path `another/path`, it returns `another/path`.

Author: kenjis

user_guide_src/source/libraries/uri/016.php

@@ -2,5 +2,5 @@
 
 $uri = new \CodeIgniter\HTTP\URI('http://www.example.com/some/path');
 
-echo $uri->getPath();   // 'some/path'
-echo $uri->setPath('another/path')->getPath();  // 'another/path'
+echo $uri->getPath();                            // '/some/path'
+echo $uri->setPath('/another/path')->getPath();  // '/another/path'