message: trivia: fix typo in README
  Remove a superfluous line of three back-ticks that caused improper formatting of the http_proxy section.
diff: @@ -36,7 +36,6 @@ An example basic invocation to build a developer version of the Envoy static bin
  ```
  The build image defaults to `envoyproxy/envoy-build-ubuntu`, but you can choose build image by setting `IMAGE_NAME` in the environment.
  -```
  In case your setup is behind a proxy, set `http_proxy` and `https_proxy` to the proxy servers before invoking the build.
  