Add a missing feature attribute to the example for std::process::Command::envs().

The person who originally wrote the example forgot to include this attribute.
This caused Travis CI to fail on commit 9b0a4a4e97 (#40794), which just fixed
formatting in the description of std::process::Command::envs().

src/libstd/process.rs

@@ -439,6 +439,8 @@ impl Command {
     /// Basic usage:
     ///
     /// ```no_run
+    /// #![feature(command_envs)]
+    ///
     /// use std::process::{Command, Stdio};
     /// use std::env;
     /// use std::collections::HashMap;