Update service_module_injector.md with correct syntax for NINJA_EXTRA config

If someone is following this tutorial, the will get a misleading NinjaExtra config in their settings.py that will silently fail and won't let DI work as expected.

Author: adriangs1996

docs/service_module_injector.md

@@ -239,7 +239,7 @@ There are different ways of registering injector Modules in a Django app.
 We register modules to `INJECTOR_MODULES` key in Ninja Extra settings in django settings.py
 
 ```python
-NinjaExtra = {
+NINJA_EXTRA = {
     'INJECTOR_MODULES': [
         'myproject.app1.modules.SomeModule',
         'myproject.app2.modules.SomeAppModule',