schema: Update instructions when modifying the schema

The instructions were out of date, since the version test now uses the
_VALID_SCHEMA_VERS array from manifest.py.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

src/west/manifest-schema.yml

@@ -13,8 +13,8 @@
 #
 #  DO NOT CHANGE THIS FILE WITHOUT:
 #
-#  - updating west.manifest.SCHEMA_VERSION
-#  - updating test_version_check_success() in test_manifest.py
+#  - updating west.manifest.SCHEMA_VERSION and 
+#    west.manifest._VALID_SCHEMA_VERS
 #  - reviewing and updating sphinx documentation
 #
 # --------------------------------------------------------------