Fix "resizing panes" up & down key mappings

Previously the README had `prefix + shift + j` for up and `prefix + shift + k` for down; it should be the other way around.

Author: Wes Moberly

README.md

@@ -43,9 +43,9 @@ a better binding for that, `prefix + a` (if your prefix is `C-a`).
 - `prefix + shift + h`<br/>
   resize current pane 5 cells to the left
 - `prefix + shift + j`<br/>
-  resize 5 cells in the up direction
-- `prefix + shift + k`<br/>
   resize 5 cells in the down direction
+- `prefix + shift + k`<br/>
+  resize 5 cells in the up direction
 - `prefix + shift + l`<br/>
   resize 5 cells to the right