docs: Fix default options for fs.createWriteStream()

The documentation for createWriteStream() references an
'encoding' property that has a default value of null. However,
this property is never referenced by createWriteStream() or
WritableState(). Instead a 'defaultEncoding' property is
referenced in WritableState() with a default of 'utf8' if no value
is supplied.

This fix updates the documentation to rename the 'encoding'
property to 'defaultEncoding' and indicate its default value of
'utf8'.

Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: https://github.com/joyent/node/pull/25591