doc-rst: parse-headers: remove trailing spaces

The function that replace references add a "\ " at the end of
references, to avoid the ReST markup parser to not identify
them as references. That works fine except for the end of lines,
as a sequence of { '\', ' ', '\n' } characters makes Sphinx
to ignore the end of line. So, strip those escape/spaces at the
end of lines.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>