Clarify about the env var for backtrace

Author: carols10cents

_posts/2017-04-27-Rust-1.17.md

@@ -284,7 +284,7 @@ stack backtrace:
   11: _start
 ```
 
-By default. As the message says, you can set an environment variable to get the full
+By default. You can set the environment variable `RUST_BACKTRACE=full` to get the full
 backtrace. We may be able to do more cleanup in the future; see [this bug](https://github.com/rust-lang/rust/pull/40264) for more.
 
 See the [detailed release notes][notes] for more.