README: fix pbkdf2 spelling, resolves #20

Author: dcousens

README.md

@@ -10,8 +10,8 @@ This library provides the functionality of PBKDF2 with the ability to use any su
 ## Usage
 
 ```
-var pbkd2f = require('pbkd2f')
-var derivedKey = pbkd2f.pbkdf2Sync('password', 'salt', 1, 32, 'sha512')
+var pbkdf2 = require('pbkdf2')
+var derivedKey = pbkdf2.pbkdf2Sync('password', 'salt', 1, 32, 'sha512')
 
 ...
 ```
@@ -21,4 +21,4 @@ var derivedKey = pbkd2f.pbkdf2Sync('password', 'salt', 1, 32, 'sha512')
 
 This module is a derivative of https://github.com/cryptocoinjs/pbkdf2-sha256/, so thanks to [JP Richardson](https://github.com/cryptocoinjs/pbkdf2-sha256/) for laying the ground work.
 
-Thank you to [FangDun Cai](https://github.com/fundon) for donating the package name on npm, if you're looking for his previous module it is located at [fundon/pbkdf2](https://github.com/fundon/pbkdf2).
+Thank you to [FangDun Cai](https://github.com/fundon) for donating the package name on npm, if you're looking for his previous module it is located at [fundon/pbkdf2](https://github.com/fundon/pbkdf2).