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: NJonathan Neuschäfer <j.neuschaefer@gmx.net>
Link: https://lore.kernel.org/r/20200308210935.7273-1-j.neuschaefer@gmx.netSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/binfmt-misc.rst

-Kernel Support for miscellaneous (your favourite) Binary Formats v1.1
-=====================================================================
+Kernel Support for miscellaneous Binary Formats (binfmt_misc)
+=============================================================
 
 This Kernel feature allows you to invoke almost (for restrictions see below)
 every program by simply typing its name in the shell.