docs: filesystems: link ubifs-authentication.rst without .rst extension

Specifying the .rst extension doesn't cause any problems AFAICT, but
it's uncommon.
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20201108132415.1789142-1-j.neuschaefer@gmx.netSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/index.rst

@@ -113,7 +113,7 @@ Documentation for filesystem implementations.
    sysv-fs
    tmpfs
    ubifs
-   ubifs-authentication.rst
+   ubifs-authentication
    udf
    virtiofs
    vfat