Make command description imperative statement, not third-person present.

In several of the text messages, the tense of the verb is inconsistent.
For example, "Add" vs "Creates".  It is customary to use imperative for
command description.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-clone.txt b/Documentation/git-clone.txt
index 644bf12..ac938d3 100644
--- a/Documentation/git-clone.txt
+++ b/Documentation/git-clone.txt
@@ -3,7 +3,7 @@
 
 NAME
 ----
-git-clone - Clones a repository into a new directory
+git-clone - Clone a repository into a new directory
 
 
 SYNOPSIS