fix(iOS): avoid cutting-off text on bottom bar buttons on iPad.

For some reason, this change resolves the issue in the attached snapshots.

When `centered=YES`, setting the text alignment to `NSTextAlignmentCenter`
may incidentally cause the text layout engine to more effectively manage the
available space. Doesn't seem like it affects anything in iPhones.

Overall, it seems like a bug in UIKit..

lib/ios/RNNTabBarItemCreator.m

@@ -97,7 +97,7 @@ - (void)appendTitleAttributes:(UITabBarItem *)tabItem
                     fontSize:fontSize
                   fontWeight:fontWeight
                        color:textColor
-                    centered:NO];
+                    centered:YES];
     [self setTitleAttributes:tabItem titleAttributes:normalAttributes];
 
     NSDictionary *selectedAttributes = [RNNFontAttributesCreator
@@ -106,7 +106,7 @@ - (void)appendTitleAttributes:(UITabBarItem *)tabItem
                     fontSize:fontSize
                   fontWeight:fontWeight
                        color:selectedTextColor
-                    centered:NO];
+                    centered:YES];
     [self setSelectedTitleAttributes:tabItem selectedTitleAttributes:selectedAttributes];
 }