[readme] Auto install not exist version in zsh .nvmrc script

Fixes nvm-sh#1272.

Author: PeterDaveHello

README.markdown

@@ -323,7 +323,7 @@ load-nvmrc() {
     local nvmrc_node_version=$(nvm version "$(cat "${nvmrc_path}")")
 
     if [ "$nvmrc_node_version" != "N/A" ] && [ "$nvmrc_node_version" != "$node_version" ]; then
-      nvm use 
+      nvm install
     fi
   elif [ "$node_version" != "$(nvm version default)" ]; then
     echo "Reverting to nvm default version"