Fix font name in styling.md (#3003)

docs/docs/styling.md

       text: 'Title',
       fontSize: 14,
       color: 'red',
-      fontFamily: 'Halvetica',
+      fontFamily: 'Helvetica',
       component: 'example.CustomTopBarTitle',
       componentAlignment: 'center',
     },
       text: 'Title',
       fontSize: 14,
       color: 'red',
-      fontFamily: 'Halvetica',
+      fontFamily: 'Helvetica',
       alignment: 'center'
     },
     background: {
     backgroundColor: 'white',
     tabColor: 'red',
     selectedTabColor: 'blue',
-    fontFamily: 'Halvetica',
+    fontFamily: 'Helvetica',
     fontSize: 10
   },
   bottomTab: {