Upper case error -> lower case.

Author: bartchr808

include/swift/AST/DiagnosticsSema.def

@@ -2734,7 +2734,7 @@ ERROR(differentiable_attr_unsupported_req_kind,none,
 ERROR(differentiable_attr_class_unsupported,none,
       "class members cannot be marked with '@differentiable'", ())
 ERROR(differentiable_attr_stored_property_unsupported,none,
-      "Stored properties cannot be marked with '@differentiable'", ())
+      "stored properties cannot be marked with '@differentiable'", ())
 NOTE(protocol_witness_missing_specific_differentiable_attr,none,
      "candidate is missing attribute '%0'", (StringRef))