git p4 doc: fix branch detection example

Make sure that the example on how to use git-p4.branchList
works if typed directly.  In particular, it does not make sense
to set a config variable until the git repository has been
initialized.

Reported-by: Olivier Delalleau <shish@keba.be>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-p4.txt

@@ -393,8 +393,10 @@ the path elements in the p4 repository.  The example above relied on the
 presence of the p4 branch.  Without p4 branches, the same result will
 occur with:
 ----
+git init depot
+cd depot
 git config git-p4.branchList main:branch1
-git p4 clone --detect-branches //depot@all
+git p4 clone --detect-branches //depot@all .
 ----