Update branch references in README.pod

[cfa]

Replace references to master with main.

https://dev.azure.com links left unchanged for now, as apparently main isn't currently being built there.

[cfa]

Regarding Azure, compare

• https://dev.azure.com/Raku/nqp/_apis/build/status/Raku.nqp?branchName=master
• https://dev.azure.com/Raku/nqp/_apis/build/status/Raku.nqp?branchName=main

[lizmat]

Thanks!