Update build instructions

[aral]

With Go version 1.16.2, on macOS, the build instructions in the readme include an erroneous third step and step two creates the binary in $GOPATH/bin/pebble. And no $GOPATH/src folder is created.

[warmy54]

This was also the case on linux

[consideRatio]

This is possible resolved via https://github.com/letsencrypt/pebble/pull/389.