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.

src/libstd/process.rs

@@ -437,7 +437,10 @@ pub fn env<K, V>(&mut self, key: K, val: V) -> &mut Command
     /// # Examples
     ///
     /// Basic usage:
+    ///
     /// ```no_run
+    /// #![feature(command_envs)]
+    ///
     /// use std::process::{Command, Stdio};
     /// use std::env;
     /// use std::collections::HashMap;