Fixes #32 - error prone zsh syntax

The old syntax started erroring with zsh 5.4.1.  This new change is confirmed working.  Note that the function used does need to be declared (it seems) before the others.

README.md

@@ -44,11 +44,7 @@ Enabling automatic switching consist in a little shell script for check if `pack
 Add the follow snippet in a file loaded by your shell agent, for example, `.extra`.
 
 ```bash
-cd () { builtin cd "$@" && chpwd; }
-pushd () { builtin pushd "$@" && chpwd; }
-popd () { builtin popd "$@" && chpwd; }
-
-chpwd () {
+function chpwd() {
   local PKG
   PKG=$PWD/package.json
   if [ -f "$PKG" ] && [ "$NODENGINE_LAST_DIR" != "$PWD" ]; then
@@ -57,6 +53,10 @@ chpwd () {
     NODENGINE_LAST_DIR=$PWD
   fi
 }
+
+function cd() { builtin cd "$@" && chpwd; }
+function pushd() { builtin pushd "$@" && chpwd; }
+function popd() { builtin popd "$@" && chpwd; }
 ```
 
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