feat: Correct casing for en-sg locale name (#1048)

Author: tarasbobak

src/locale/en-SG.js src/locale/en-sg.js

@@ -2,7 +2,7 @@
 import dayjs from 'dayjs'
 
 const locale = {
-  name: 'en-SG',
+  name: 'en-sg',
   weekdays: 'Sunday_Monday_Tuesday_Wednesday_Thursday_Friday_Saturday'.split('_'),
   months: 'January_February_March_April_May_June_July_August_September_October_November_December'.split('_'),
   weekStart: 1,