doc: fix arg definition in fs

[apapirovski]

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

Here is the error:

Warning: invalid param "len "
 > {"textRaw":"`len` {integer} **Default:** `0` ","name":"len","type":"integer","desc":"**Default:** `0`"}
 > filehandle.truncate(len = 0)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

[apapirovski]

Lite CI: https://ci.nodejs.org/job/node-test-pull-request-lite/164/

[apapirovski]

Thanks everyone! Landed in dbd1d1d

[MylesBorins]

Should this be backported to v9.x-staging? If yes please follow the guide and raise a backport PR, if not let me know or add the dont-land-on label.

[targos]

This depends on #18297 which won't land on older branches.