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>
diff --git a/tools/patman/patman.rst b/tools/patman/patman.rst
index a8b317e..e013558 100644
--- a/tools/patman/patman.rst
+++ b/tools/patman/patman.rst
@@ -280,7 +280,7 @@
 
 Commit-notes:
     Similar, but for a single commit (patch). These notes will appear
-    immediately below the --- cut in the patch file::
+    immediately below the ``---`` cut in the patch file::
 
         Commit-notes:
         blah blah