update name of magic string used

update name of magic string used to correctly reflect code

Author: tylercaceres

content/recipes/sql-typeorm.md

@@ -111,7 +111,7 @@ export const photoProviders = [
 ];
 ```
 
-> warning **Warning** In the real-world applications you should avoid **magic strings**. Both `PHOTO_REPOSITORY` and `DATABASE_CONNECTION` should be kept in the separated `constants.ts` file.
+> warning **Warning** In the real-world applications you should avoid **magic strings**. Both `PHOTO_REPOSITORY` and `DATA_SOURCE` should be kept in the separated `constants.ts` file.
 
 Now we can inject the `Repository<Photo>` to the `PhotoService` using the `@Inject()` decorator: