change 'popoverVisible' to more closely match UIPopoverController API

Author: hsoi

WYPopoverController/WYPopoverController.h

@@ -99,7 +99,7 @@ typedef NS_OPTIONS(NSUInteger, WYPopoverAnimationOptions) {
 @property (nonatomic, copy) NSArray                         *passthroughViews;
 @property (nonatomic, assign) BOOL                           wantsDefaultContentAppearance;
 @property (nonatomic, assign) UIEdgeInsets                   popoverLayoutMargins;
-@property (nonatomic, assign, readonly) BOOL                 isPopoverVisible;
+@property (nonatomic, assign, readonly, getter=isPopoverVisible) BOOL popoverVisible;
 @property (nonatomic, strong, readonly) UIViewController    *contentViewController;
 @property (nonatomic, assign) CGSize                         popoverContentSize;
 @property (nonatomic, assign) CGFloat                        animationDuration;