Fix readme settings example (#1910)

Author: Viicos

README.md

@@ -305,7 +305,7 @@ So, mypy would not like this code:
 ```python
 from django.conf import settings
 
-settings.CUSTOM_VALUE  # E: 'Settings' object has no attribute 'CUSTOM_SETTING'
+settings.CUSTOM_VALUE  # E: 'Settings' object has no attribute 'CUSTOM_VALUE'
 ```
 
 To handle this corner case we have a special setting `strict_settings` (`True` by default),
@@ -328,7 +328,7 @@ And then:
 
 ```python
 # Works:
-reveal_type(settings.EXISTS_IN_RUNTIME)  # N: Any
+reveal_type(settings.EXISTS_AT_RUNTIME)  # N: Any
 
 # Errors:
 reveal_type(settings.MISSING)  # E: 'Settings' object has no attribute 'MISSING'