io/fs: make WalkDirFunc parameter name consistent with doc comment
io/fs: make WalkDirFunc parameter name consistent with doc comment

The the DirEntry parameter of WalkDirFunc is referred to as `d` in the doc comment.

Change-Id: Ibfcf7908eaa0ef1309898150e8fd71101e7de09b
GitHub-Last-Rev: e858c52d81b93d293621d7e744bdcb7d6cbd412c
GitHub-Pull-Request: golang/go#42447
Reviewed-on: https://go-review.googlesource.com/c/go/+/268277
Trust: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go