docs: filesystems: update netfs-api.rst reference

docs: filesystems: update netfs-api.rst reference

Changeset efc930fa1d84 ("docs: filesystems: caching/netfs-api.txt: convert it to ReST")
renamed: Documentation/filesystems/caching/netfs-api.txt
to: Documentation/filesystems/caching/netfs-api.rst.

Update its cross-reference accordingly.

Fixes: efc930fa1d84 ("docs: filesystems: caching/netfs-api.txt: convert it to ReST")
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/5f867f01d42c3e65e111167739ed1a41a26623f9.1656234456.git.mchehab@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>