Readme: clarify Mongo install usage

Author: nathanleiby

README.md

@@ -51,11 +51,11 @@ Runs [report-card](https://github.com/clever/report-card).
 $ ./circleci/report-card [DOCKER_USER] [DOCKER_PASS] [DOCKER_EMAIL] [GITHUB_TOKEN]
 ```
 
-### Mongo install (version 2.4 only)
+### Mongo install
 
-Installs Mongo version 2.4, rather than the default version in CircleCI.
-At time of writing, `v3.0.7` was default version in [Ubuntu 14.04 (Trusty) image](https://circleci.com/docs/build-image-trusty/#mongodb).
+Installs a specific Mongo version, rather than the default version in CircleCI.
+At time of writing, `v3.0.7` was default version in CircleCI's [Ubuntu 14.04 (Trusty) image](https://circleci.com/docs/build-image-trusty/#mongodb).
 
 ```
-$ ./circleci/mongo-install-2.4
+$ ./circleci/mongo-install [VERSION]
 ```