modify "2.1. Common Options - xcode_dir" description

`/usr/bin/xcode-select -printPath` returns below.

```
xcode-select: Error: unknown command option '-printPath'.

xcode-select: Report or change the path to the active
              Xcode installation for this machine.

Usage: xcode-select --print-path
           Prints the path of the active Xcode folder
   or: xcode-select --switch <xcode_path>
           Sets the path for the active Xcode folder
   or: xcode-select --version
           Prints the version of xcode-select
```

and `/usr/bin/xcode-select --print-path` returns right value.