doc: use AIX instead of Aix in fs.md

Both for consistency within the doc and to conform with IBM's usage
regarding their on product, refer to the operating system as AIX rather
than Aix.

PR-URL: https://github.com/nodejs/node/pull/38535


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>