feat: improve wording of message OPF-025

Fix #959

src/main/resources/com/adobe/epubcheck/messages/MessageBundle.properties

@@ -181,7 +181,7 @@ OPF_018=The 'remote-resources' property was declared in the OPF, but no referenc
 OPF_019=Spine tag was not found in the OPF file.
 OPF_020=Excessive number of spine items.
 OPF_021=Use of non-registered URI scheme type in href: '%1$s'.
-OPF_025=Property '%1$s' can take only one value.
+OPF_025=Property value list '%1$s' is not allowed, only one value must be specified.
 OPF_026=Found malformed property value: '%1$s'.
 OPF_027=Undefined property: '%1$s'.
 OPF_028=Undeclared prefix: '%1$s'.