Minor readme update

Author: Sijo Cheeran

README.md

@@ -12,7 +12,7 @@ npm install testcafe-browser-provider-crossbrowsertesting
 ## Usage
 Before using this plugin, save the CrossBrowserTesting username and auth key to environment variables `CBT_TUNNELS_USERNAME` and `CBT_TUNNELS_AUTHKEY`.
 
-Setting Environment Variables for Mac OS X/Linux
+## Setting Environment Variables for Mac OS X/Linux
 In Terminal mode, enter vi ~/.bash_profile, and then press Enter.
 Press i to insert text into your profile file.
 Enter these lines: