Fix AppVeyor URL in README.md
Fix AppVeyor URL in README.md

My AppVeyor account doesn't rebuild cppcodec when new changes are being
pushed to master.

Use efidler's AppVeyor URL instead. As tplgy admin, he has kindly added
AppVeyor integration to the upstream project and thus, updates get pushed
to his account.
jpetso/cppcodec,tplgy/cppcodec