Documentation/git-merge.txt: use backticks for command wrapping

Documentation/git-merge.txt: use backticks for command wrapping

As René found in the guidance from CodingGuidelines:

   Literal examples (e.g. use of command-line options, command names,
   branch names, URLs, pathnames (files and directories), configuration
   and environment variables) must be typeset in monospace (i.e. wrapped
   with backticks)

So all instances of single and double quotes for wraping said examples
were replaced with simple backticks.

Suggested-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>