docs(stats/halfnormal): fix require path in README example

Author: anujmishra03

lib/node_modules/@stdlib/stats/base/dists/halfnormal/README.md

@@ -5,6 +5,7 @@
 ## Usage
 
 ```javascript
-var pdf = require('@stdlib/stats/base/dists/halfnormal/lib/pdf');
+var pdf = require( '@stdlib/stats/base/dists/halfnormal' ).pdf;
+
 var y = pdf( 1.0, 1.0 );
 console.log( y );