docs: filesystems: convert hfsplus.txt to ReST

Just trivial changes:

- Add a SPDX header;
- Add it to filesystems/index.rst.

While here, adjust document title, just to make it use the same
style of the other docs.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/429840[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/filesystems/hfsplus.txt Documentation/filesystems/hfsplus.rst

···
+.. SPDX-License-Identifier: GPL-2.0
 
+======================================
 Macintosh HFSPlus Filesystem for Linux
 ======================================
 

Documentation/filesystems/index.rst

···
    f2fs
    gfs2
    gfs2-uevents
+   hfsplus
    fuse
    overlayfs
    virtiofs