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>

doc/develop/sending_patches.rst

@@ -121,7 +121,7 @@ General Patch Submission Rules
    * For new features: a description of the feature and your implementation.
 
 * Additional comments which you don't want included in U-Boot's history can be
-  included below the first "---" in the message body.
+  included below the first "``---``" in the message body.
 
 * If your description gets too long, that's a strong indication that you should
   split up your patch.
@@ -253,7 +253,7 @@ to observe the following rules.
 
 * Please make sure to keep a "change log", i.e. a description of what you have
   changed compared to previous versions of this patch. This change log should
-  be added below the "---" line in the patch, which starts the "comment
+  be added below the "``---``" line in the patch, which starts the "comment
   section", i.e. which contains text that does not get included into the
   actual commit message.
   Note: it is *not* sufficient to provide a change log in some cover letter

tools/patman/patman.rst

@@ -280,7 +280,7 @@ Series-notes:
 
 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