doc: patch-id: spell out the git-diff-tree(1) form

You specifically need `--patch` since the default output is a raw diff.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: LemmingAvalanche

Documentation/git-patch-id.adoc

@@ -21,7 +21,7 @@ the same time also reasonably unique, i.e., two patches that have the same
 
 The main usecase for this command is to look for likely duplicate commits.
 
-When dealing with `git diff-tree` output, it takes advantage of
+When dealing with `git diff-tree --patch` output, it takes advantage of
 the fact that the patch is prefixed with the object name of the
 commit, and outputs two 40-byte hexadecimal strings.  The first
 string is the patch ID, and the second string is the commit ID.