doc: fix three-dash references in reStructuredText

doc: fix three-dash references in reStructuredText

The "Sending patches" and "Patman patch manager" documents refer to the
three-dash, "---", that separates patch parts. The symbol is written in
the documentation in the same form as it is in actual patches: as three
U+002D HYPHEN-MINUS characters. When building the documentation, Sphinx
converts this symbol to a U+2014 EM DASH, that is invalid in this case.

Make the three-dash symbol a reStructuredText inline literal to prevent
its conversion; that is, enclose it in a pair of double backquotes (`).

Signed-off-by: Grzegorz Szymaszek <gszymaszek@short.pl>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>