docs: Quote PWD in window title example (starship#2539)

Quote expansion of $PWD to prevent word-splitting of its value, which
gives an incorrect result if there is a single space and an error
message if there is more than one space:

    ❯ basename /tmp/foo
    foo

    ❯ basename /tmp/foo bar
    foo

    ❯ basename /tmp/foo bar baz
    basename: extra operand ‘baz’
    Try 'basename --help' for more information.

docs/advanced-config/README.md

@@ -80,7 +80,7 @@ add the following snippet to your `~/.bashrc` or `~/.zshrc`:
 
 ```bash
 function set_win_title(){
-    echo -ne "\033]0; $(basename $PWD) \007"
+    echo -ne "\033]0; $(basename "$PWD") \007"
 }
 starship_precmd_user_func="set_win_title"
 ```