Add whitespace to fix typo

Otherwise if someone copy+paste this it will throw this error.
ERROR Error while reading config: Invalid YAML syntax:

mapping values are not allowed here
  in "nspanel.yaml", line 25, column 13:

docs/en/customization.md

@@ -253,7 +253,7 @@ You can easily force a timezone by adding this to your ESPHome settings:
 
 ```yaml
 time:
-  - id:!extend time_provider
+  - id: !extend time_provider
     timezone: "America/Cancun"
 ```