add owner @open-telemetry/specs-semconv-maintainers to semantic-conve…

…ntions owners (#287)

Co-authored-by: Armin Ruech <7052238+arminru@users.noreply.github.com>
Co-authored-by: Joao Grassi <5938087+joaopgrassi@users.noreply.github.com>

.github/CODEOWNERS

@@ -16,4 +16,4 @@
 *   @open-telemetry/specs-approvers
 
 # Owners for the semantic conventions generator tool
-/semantic-conventions/      @open-telemetry/specs-approvers @open-telemetry/specs-semconv-approvers
+/semantic-conventions/      @open-telemetry/specs-approvers @open-telemetry/specs-semconv-approvers @open-telemetry/specs-semconv-maintainers