doc: buffer, adding missing backtick

PR-URL: nodejs/node-v0.x-archive#25811
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/buffer.markdown

@@ -250,7 +250,7 @@ may be beyond the end of the buffer. Defaults to `false`.
 
 Decodes and returns a string from buffer data encoded using the specified
 character set encoding. If `encoding` is `undefined` or `null`, then `encoding`
-defaults to `'utf8'. The `start` and `end` parameters default to `0` and
+defaults to `'utf8'`. The `start` and `end` parameters default to `0` and
 `buffer.length` when `undefined`.
 
     buf = new Buffer(26);