fix(c#): yymmdd function

[ttodua]

in all other langs, the infix arg for yymmdd is empty string by default, eg:

ccxt/ts/src/base/functions/time.ts

Line 131 in ef2195e

const yymmdd = (timestamp, infix = '') => ymd (timestamp, infix, false);

causing errors in c#

[ttodua]

@carlosmiei ^