general: Apply submodules' path change in doc and zprezto-update

We need to accommodate cases where a submodule local path (or remote)
URL can change. Accordingly, update instruction and `zprezto-update`
function to synchronizes submodules' remote URL configuration setting
to the updated value automatically.

README.md

@@ -65,6 +65,7 @@ To pull the latest changes and update submodules manually:
 ```console
 cd $ZPREZTODIR
 git pull
+git submodule sync --recursive
 git submodule update --init --recursive
 ```
 

init.zsh

@@ -44,6 +44,7 @@ function zprezto-update {
         printf "There is an update available. Trying to pull.\n\n"
         if git pull --ff-only; then
           printf "Syncing submodules\n"
+          git submodule sync --recursive
           git submodule update --init --recursive
           return $?
         else