Remove duplicate month symbol in korean

Previous format `'%Y년 %B월 %d일, %H시 %M분 %S초 %Z'` renders:

2014년 **6월월** 12일, 12시 30분 00초 +09:00

In this case, the 월 char is incorrectly printed twice. By changing %B to %m we can avoid this.
Merge pull request #454 from johnnyshields/patch-1

Remove duplicate month symbol in Korean