docs: fix typos in filesystem API reference

[Ankit-Mohanty-23]

Summary

Fixed two typos in docs/api/filesystem.md:

1. Spelling error in getStats description

• existance → existence

2. Wrong property name in setPermissions code example

• otherAll → othersAll
• All other permission properties use the others prefix
  (othersRead, othersWrite, othersExec, othersAll).
  The example used otherAll which silently fails at runtime.

Both are in docs/api/filesystem.md.

[Ankit-Mohanty-23]

I'd like to work on this. Will submit a PR shortly.