update README

fix error when the full path does not exist.

README.md

@@ -146,7 +146,7 @@ This configuration has been tested on Mac & Linux. Windows is supported through
 
 ```bash
 # Create your project directory then go into it:
-mkdir ~/Sites/magento
+mkdir -p ~/Sites/magento
 cd $_
 
 # Run this automated one-liner from the directory you want to install your project.
@@ -174,7 +174,7 @@ Same result as the one-liner above. Just replace `magento.test` references with
 
 ```bash
 # Create your project directory then go into it:
-mkdir ~/Sites/magento
+mkdir -p ~/Sites/magento
 cd $_
 
 # Download the Docker Compose template:
@@ -203,7 +203,7 @@ open https://magento.test
 bin/mysqldump > ~/Sites/existing/magento.sql
 
 # Create your project directory then go into it:
-mkdir ~/Sites/magento
+mkdir -p ~/Sites/magento
 cd $_
 
 # Download the Docker Compose template: