doc: show keylen in pbkdf2 as a byte length

Ensure that keylen for pbkdf2 is documented as a length of bytes and not
bits.

PR-URL: #3334
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/api/crypto.markdown

@@ -621,13 +621,13 @@ Example (obtaining a shared secret):
 ## crypto.pbkdf2(password, salt, iterations, keylen[, digest], callback)
 
 Asynchronous PBKDF2 function.  Applies the selected HMAC digest function
-(default: SHA1) to derive a key of the requested length from the password,
+(default: SHA1) to derive a key of the requested byte length from the password,
 salt and number of iterations.  The callback gets two arguments:
 `(err, derivedKey)`.
 
 Example:
 
-    crypto.pbkdf2('secret', 'salt', 4096, 512, 'sha256', function(err, key) {
+    crypto.pbkdf2('secret', 'salt', 4096, 64, 'sha256', function(err, key) {
       if (err)
         throw err;
       console.log(key.toString('hex'));  // 'c5e478d...1469e50'