git: Use `update` instead of `foreach` in alias 'gSu'

Modern `git` can use `submodule` option `update` instead of `foreach`
to achieve approximately similar outcome.

To allows call without assuming that all submodules will have default
branch as `master`, the preferred approach would be to call
`git submodule update --remote --recursive` after registering the
correct branch for submodule in `.gitmodules`:

```
git config -f .gitmodules submodule.<path>.branch <branch>
```

For more discussion, see: https://stackoverflow.com/a/33835815

Signed-off-by: Indrajit Raychaudhuri <irc@indrajit.com>

modules/git/alias.zsh

@@ -252,7 +252,7 @@ if ! zstyle -t ':prezto:module:git:alias' skip; then
   alias gSl='git submodule status'
   alias gSm='git-submodule-move'
   alias gSs='git submodule sync'
-  alias gSu='git submodule foreach git pull origin master'
+  alias gSu='git submodule update --remote --recursive'
   alias gSx='git-submodule-remove'
 
   # Tag (t)