CustomCalloutView: Fix int-conversion warning

> incompatible pointer to integer conversion returning 'id _Nullable' from a function with result type 'BOOL' (aka 'signed char') [-Wint-conversion]

Examples/ObjectiveC/CustomCalloutView.m

@@ -130,7 +130,7 @@ - (void)setCenter:(CGPoint)center {
 - (BOOL)isCalloutTappable
 {
     if ([self.delegate respondsToSelector:@selector(calloutViewShouldHighlight:)]) {
-        return [self.delegate performSelector:@selector(calloutViewShouldHighlight:) withObject:self];
+        return (BOOL)[self.delegate performSelector:@selector(calloutViewShouldHighlight:) withObject:self];
     }
 
     return NO;