Update CODEOWNERS

Inline with finos#893 we want to ensure that members of calm-schema-governance have approved changes to the calm schema directory

Author: rocketstack-matt

.github/CODEOWNERS

@@ -1,6 +1,6 @@
 * @finos/architecture-as-code-maintainers
 
-/calm/ @finos/architecture-as-code-maintainers
+/calm/ @finos/calm-schema-governance
 
 /calm-hub/ @jpgough-ms @rocketstack-matt @grahampacker-ms @Thels