MapWithAILayerInfo: Fix typo

Call was to NetworkManager#isOffline, but was stored in a variable named
"online". Fix: Invert the returned boolean from NetworkManager#isOffline

Signed-off-by: Taylor Smock <tsmock@fb.com>

src/main/java/org/openstreetmap/josm/plugins/mapwithai/data/mapwithai/MapWithAILayerInfo.java

@@ -309,7 +309,7 @@ public class MapWithAILayerInfo {
         }
 
         protected void loadSource(String source) {
-            boolean online = NetworkManager.isOffline(source);
+            boolean online = !NetworkManager.isOffline(source);
             if (clearCache && online) {
                 CachedFile.cleanup(source);
             }