Fix link to NIOFileSystem from NIO index page

[rnro]

Fix link to NIOFileSystem from NIO index page

Motivation:

The existing link to NIOFileSystem docs 404's

Modifications:

Speculative fixes because getting any links to work locally is challenging.

• rename NIOFileSystem.md -> index.md in line with other modules
• modify the link on the NIO index page to reference _NIOFileSystem local experiments suggest it might change the form of the generated URL to where the docs can be found (https://swiftpackageindex.com/apple/swift-nio/2.67.0/documentation/_NIOFileSystem).

Result:

Hopefully the NIOFileSystem docs link will work