docs/interpret-trailers: fix agreement error

In the description of git interpret-trailers, we describe "a group…of
lines" that have certain characteristics.  Ensure both options
describing this group use a singular verb for parallelism.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-interpret-trailers.txt

@@ -48,7 +48,7 @@ with only spaces at the end of the commit message part, one blank line
 will be added before the new trailer.
 Existing trailers are extracted from the input message by looking for
-a group of one or more lines that (i) are all trailers, or (ii) contains at
+a group of one or more lines that (i) is all trailers, or (ii) contains at
 least one Git-generated or user-configured trailer and consists of at
 least 25% trailers.
 The group must be preceded by one or more empty (or whitespace-only) lines.