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

aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust