doc/go1.21: document that os.DirFS implements fs.Read{File,Dir}FS

doc/go1.21: document that os.DirFS implements fs.Read{File,Dir}FS

Also add a missing </a> in the preceding section.

For #53761

Change-Id: I8e64b86b5b32067f954d58cf9adf86cb4d2eeb2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/499735
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Auto-Submit: Eli Bendersky <eliben@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
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