docs: fs: proc.rst: fix a warning due to a merge conflict

Changeset 424037b7 ("mm: smaps: Report arm64 guarded pages in smaps")
added a new parameter to a table. This causes Sphinx warnings,
because there's now an extra "-" at the wrong place:

	/devel/v4l/docs/Documentation/filesystems/proc.rst:548: WARNING: Malformed table.
	Text in column margin in table line 29.

	==    =======================================
	rd    readable
	...
	bt  - arm64 BTI guarded page
	==    =======================================

Fixes: 424037b7 ("mm: smaps: Report arm64 guarded pages in smaps")
Fixes: c33e97ef ("docs: filesystems: convert proc.txt to ReST")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/28c4f4c5c66c0fd7cbce83fe11963ea6154f1d47.1591137229.git.mchehab+huawei@kernel.org


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