Fixes heading in README.md

Author: neatc0der

README.md

@@ -112,7 +112,7 @@ extra_javascript:
   - js/markmap-view.js
 ```
 
-### Usage of proxy is prevent download of javascript files
+### Usage of proxy is preventing download of javascript files
 
 Usually proxies should be supported by `requests`, which is used for downloading all required javascript files. If the issue remains, try downloading the files yourself and store them accordingly (see [Static javascript files not working](#static-javascript-files-not-working))