doc: fix arg definition in fs

Currently doc building doesn't support ES-style default params in
function definitions which causes an error.

PR-URL: #18678
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Gus Caplan <me@gus.host>

Author: apapirovski

doc/api/fs.md

@@ -3388,7 +3388,7 @@ added: REPLACEME
 Asynchronous fsync(2). The `Promise` is resolved with no arguments upon
 success.
 
-#### filehandle.truncate(len = 0)
+#### filehandle.truncate(len)
 <!-- YAML
 added: REPLACEME
 -->