fix day number not centered vertically (#2207)

Author: haeniya

src/calendar/day/basic/style.ts

@@ -13,7 +13,8 @@ export default function styleConstructor(theme: Theme = {}) {
     base: {
       width: 32,
       height: 32,
-      alignItems: 'center'
+      alignItems: 'center',
+      justifyContent: 'center'
     },
     text: {
       marginTop: constants.isAndroid ? 4 : 6,