docs: Update example config to remove old var (#1546)

Have updated the example toml config to remove an old var and replace it
with the updated replacement.

docs/config/README.md

@@ -17,7 +17,7 @@ All configuration for starship is done in this [TOML](https://github.com/toml-la
 
 ```toml
 # Don't print a new line at the start of the prompt
-add_newline = false
+format = "$all"
 
 # Replace the "❯" symbol in the prompt with "➜"
 [character]                            # The name of the module we are configuring is "character"