doc: change encoding to decoding

As per the example, `querystring.parse` actually does URL decoding, not
encoding.

PR-URL: https://github.com/nodejs/node/pull/15706
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

doc/api/deprecations.md

@@ -675,7 +675,7 @@ querystring.parse(str, '\n', '=');
 ```
 
 *Note*: This function is not completely equivalent to `querystring.parse()`. One
-difference is that `querystring.parse()` does url encoding:
+difference is that `querystring.parse()` does url decoding:
 
 ```sh
 > querystring.parse('%E5%A5%BD=1', '\n', '=');