Format badssl.test in README as code.

Author: lgarron

README.md

@@ -23,7 +23,7 @@ Your user should be part of the `docker` group or otherwise permitted to access
     make list-hosts # list of domains to copy into /etc/hosts
     make docker
 
-Now you can visit badssl.test in your browser.
+Now you can visit `badssl.test` in your browser.
 The root CA is at `certs/sets/test/gen/crt/ca-root.crt`. If you'd like to preserve it even when you run `make clean`, run:
 
     cd certs/sets/test