Update admin credentials docs/example

This section is outdated and requires update: https://repo1.dso.mil/big-bang/product/packages/keycloak/-/blob/main/docs/configuration.md?ref_type=heads#admin-user

secrets:
-        credentials:
+        env:
           stringData:
-            adminuser: your_admin_username
-            password: your_admin_password
+            KEYCLOAK_ADMIN: your_admin_username
+            KEYCLOAK_ADMIN_PASSWORD: your_admin_password