Update instructions with preferred flavor of wpad

[jfly]

This fixes https://github.com/awilliams/wifi-presence/issues/20

wpad-*-wolfssl still exists, but it's not installed by default anymore. In https://github.com/openwrt/openwrt/commit/2630e5063df0240eb0d05cd6e953b2bd412e3215, the default changed to wpad-basic-mbedtls. It sounds like that might be a better choice, so I opted to tell people to wpad-mbedtls as well.