Documentation: filesystems: automount-support: Change reference to document autofs.txt to autofs.rst

This patch fixes following documentation build warning:
Warning: Documentation/filesystems/automount-support.txt references
a file that doesn't exist: Documentation/filesystems/autofs.txt
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Link: https://lore.kernel.org/r/20191204101939.6939-1-madhuparnabhowmik04@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>