scripts/rfc3339date.lua: fix pattern validity

scripts/rfc3339date.lua

 local time = os.time()
 local timestamp = os.date('%F %T', time)
-local timezone = os.date('%z', time):gsub('^([+-]%d%d)(%d%d)$', '%1:%2')
+local timezone = os.date('%z', time):gsub('^([%+%-]%d%d)(%d%d)$', '%1:%2')
 
 print(timestamp .. timezone)