[introspection] Fix ApiAvailabilityTest macOS max version

tests/introspection/ApiAvailabilityTest.cs

@@ -54,7 +54,7 @@ namespace Introspection {
 			};
 #else
 			Minimum = new Version (10,7);
-			Maximum = new Version (10,13,2); // setting OSX_SDK_VERSION to 10.13.2 (instead of 10.13 breaks other assumptions)
+			Maximum = new Version (10,13,4); // setting OSX_SDK_VERSION to 10.13.4 (instead of 10.13 breaks other assumptions)
 			Filter = (AvailabilityBaseAttribute arg) => {
 				return (arg.AvailabilityKind != AvailabilityKind.Introduced) || (arg.Platform != PlatformName.MacOSX);
 			};