doc: 'trailers' is the preferred way to format trailers

The documentation makes reference to 'contents:trailers' as an example
to dig the trailers out of a commit. 'trailers' is an unmentioned
alternative, which is treated as an alias of 'contents:trailers'.

Since 'trailers' is easier to type, prefer that as the designated way to
dig out trailers information.

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

Author: ttaylorr

Documentation/git-for-each-ref.txt

@@ -217,7 +217,8 @@ 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 '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`).