doc: update emitClose default for fs streams

The default for the `emitClose` option was changed from `false` to
`true` by nodejs/node#31408 which landed in f0d2df41

 for v14.0.0.
This commit updates the fs doc to match the current behavior.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

PR-URL: https://github.com/nodejs/node/pull/36653


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>