Doc: git.txt: remove backticks from link and add git-scm.com/docs

While checking the html formatted git(1) manual page, it was noted
that the link to https://git.github.io/htmldocs/git.html was formatted
as code. Remove the backticks.

While at it, add the https://git-scm.com/docs link which one reviewer
noted had linkable section headings.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -33,7 +33,8 @@ individual Git commands with "git help command".  linkgit:gitcli[7]
 manual page gives you an overview of the command-line command syntax.
 
 A formatted and hyperlinked copy of the latest Git documentation
-can be viewed at `https://git.github.io/htmldocs/git.html`.
+can be viewed at https://git.github.io/htmldocs/git.html
+or https://git-scm.com/docs.
 
 
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