t3904-stash-patch: fix test description

The old description is rather clearly a wrong cut-and-paste from
t2016-checkout-patch.sh.

Signed-off-by: Matthieu Moy <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: moy

t/t3904-stash-patch.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-test_description='git checkout --patch'
+test_description='stash -p'
 . ./lib-patch-mode.sh
 
 test_expect_success PERL 'setup' '