Link to cargo's `build-std` feature instead of `xargo` in custom target docs
Link to cargo's `build-std` feature instead of `xargo` in custom target docs

The `xargo` tool is in maintenance mode since 2018 and the `build-std` feature of cargo already works reasonably well for most use cases.
aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust