doc: use consistent spelling for "falsy"

We use "falsy" in multiple places in our docs, but only use "false-y" in
one place in url.md. Replace it with "falsy".

PR-URL: #34545
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Pranshu Srivastava <rexagod@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/url.md

@@ -1145,7 +1145,7 @@ changes:
   - version: v7.0.0
     pr-url: https://github.com/nodejs/node/pull/7234
     description: URLs with a `file:` scheme will now always use the correct
-                 number of slashes regardless of `slashes` option. A false-y
+                 number of slashes regardless of `slashes` option. A falsy
                  `slashes` option with no protocol is now also respected at all
                  times.
 -->