docs(custom): use single equal sign for 'when' clause of 'custom.foo' example (starship#4454)

use single equal sign for 'when' clause of 'custom.foo' config example

Author: ubunatic

docs/config/README.md

@@ -3968,7 +3968,7 @@ with shell details and starship configuration if you hit such scenario.
 [custom.foo]
 command = "echo foo" # shows output of command
 detect_files = ["foo"] # can specify filters but wildcards are not supported
-when = """ test "$HOME" == "$PWD" """
+when = """ test "$HOME" = "$PWD" """
 format = " transcending [$output]($style)"
 
 [custom.time]