Fix oshpark drill file extension.  Needs to be .XLN not .TXT

OSH Park guidelines call for a .XLN extension in their gerbers.

https://oshpark.com/guidelines
    boardname.GTL Top Layer
    boardname.GBL Bottom Layer
    boardname.GTS Top Soldermask
    boardname.GBS Bottom Soldermask
    boardname.GTO Top Silkscreen
    boardname.GBO Bottom Silkscreen
    boardname.GKO Board Outline
    boardname.G2L only if you're uploading a four layer board
    boardname.G3L only if you're uploading a four layer board
    boardname.XLN Drills

Fix only tested by visualization on the website.
No boards sent to the fab yet