Rollup merge of #40794 - s3rvac:fix-formatting-in-command-envs-docs, r=steveklabnik

Fix formatting in the docs for std::process::Command::envs()

An empty line between the *Basic usage:* text and the example is required to properly format the code. Without the empty line, the example is not formatted as code.

[Here](https://doc.rust-lang.org/std/process/struct.Command.html#method.envs) you can see the current (improper) formatting.