doc: minor grammatical update

Per: https://github.com/joyent/node/pull/9009

Originally submitted by @peerwit

Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2378

doc/api/crypto.markdown

@@ -135,7 +135,7 @@ Returned by `crypto.createHash`.
 
 Updates the hash content with the given `data`, the encoding of which
 is given in `input_encoding` and can be `'utf8'`, `'ascii'` or
-`'binary'`.  If no encoding is provided and the input is a string an
+`'binary'`.  If no encoding is provided, and the input is a string, an
 encoding of `'binary'` is enforced. If `data` is a `Buffer` then
 `input_encoding` is ignored.