[Fix] use monospace font for "upstream" keyword

See @ben's comment on PR #1444[1].

[1]: https://github.com/progit/progit2/pull/1444/files/20ec785536bb526e16dc0de8fb7ad1fe2635049c#r446307067

Author: Vipul Kumar

book/06-github/sections/2-contributing.asc

@@ -270,7 +270,7 @@ To https://github.com/tonychacon/blink
    ef4725c..3c8d735  slower-blink -> slow-blink
 ----
 
-<1> Add the original repository as a remote named ``upstream''.
+<1> Add the original repository as a remote named `upstream`.
 <2> Fetch the newest work from that remote.
 <3> Merge the main branch of that repository into your topic branch.
 <4> Fix the conflict that occurred.