doc: fix fs.read arg type

About fs.read's 2nd argument, string is invalid.

PR-URL: nodejs#12034
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/fs.md

@@ -1415,7 +1415,7 @@ changes:
 -->
 
 * `fd` {integer}
-* `buffer` {string|Buffer|Uint8Array}
+* `buffer` {Buffer|Uint8Array}
 * `offset` {integer}
 * `length` {integer}
 * `position` {integer}