doc: use "<literal>"-style quoting for literal strings

"'<string>'"-style quoting is not appropriate when quoting literal
strings in the "Documentation/" subtree.

In preparation for adding additional information to this section of
git-for-each-ref(1)'s documentation, update them to use "`<literal>`"
instead.

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

Documentation/git-for-each-ref.txt

@@ -213,12 +213,12 @@ and `date` to extract the named component.
 The complete message in a commit and tag object is `contents`.
 Its first line is `contents:subject`, where subject is the concatenation
 of all lines of the commit message up to the first blank line.  The next
-line is 'contents:body', where body is all of the lines after the first
+line is `contents:body`, where body is all of the lines after the first
 blank line.  The optional GPG signature is `contents:signature`.  The
 first `N` lines of the message is obtained using `contents:lines=N`.
 Additionally, the trailers as interpreted by linkgit:git-interpret-trailers[1]
-are obtained as 'trailers' (or by using the historical alias
-'contents:trailers').
+are obtained as `trailers` (or by using the historical alias
+`contents:trailers`).
 
 For sorting purposes, fields with numeric values sort in numeric order
 (`objectsize`, `authordate`, `committerdate`, `creatordate`, `taggerdate`).