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: NJunio C Hamano <gitster@pobox.com>

Documentation/git-archive.txt

@@ -3,7 +3,7 @@ git-archive(1)
 
 NAME
 ----
-git-archive - Creates an archive of files from a named tree
+git-archive - Create an archive of files from a named tree
 
 
 SYNOPSIS

Documentation/git-clone.txt

@@ -3,7 +3,7 @@ git-clone(1)
 
 NAME
 ----
-git-clone - Clones a repository into a new directory
+git-clone - Clone a repository into a new directory
 
 
 SYNOPSIS

Documentation/git-prune.txt

@@ -3,7 +3,7 @@ git-prune(1)
 
 NAME
 ----
-git-prune - Prunes all unreachable objects from the object database
+git-prune - Prune all unreachable objects from the object database
 
 
 SYNOPSIS