docs: admin-guide: binfmt-misc: Improve the title

Trim the title a bit, since it's relatively long. Add `binfmt_misc` to
make it easier to search for the feature by its common name.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20200308210935.7273-1-j.neuschaefer@gmx.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>