Update public-signing.md (#42129)
Update public-signing.md (#42129)

@jaredpar Updated this doc based on comments on Twitter discussion with you and @dsplaisted
ericstj/corefx,shimingsg/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx,shimingsg/corefx,shimingsg/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx