Update CONTRIBUTING.md (#292)

I hope this is the correct CLA link. Exceptionless.Net used an a link to an old service that the "parent" report Exceptionless did not use.

Author: mgnslndh

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 ## Before You Start
 
-Anyone wishing to contribute to the **[Exceptionless/Exceptionless.Net](https://github.com/exceptionless/exceptionless.net)** project **MUST read & sign the [Exceptionless Contribution License Agreement](http://www.clahub.com/agreements/exceptionless/Exceptionless)**. The Exceptionless team is legally prevented from accepting any pull requests from users who have not signed the CLA first.
+Anyone wishing to contribute to the **[Exceptionless/Exceptionless.Net](https://github.com/exceptionless/exceptionless.net)** project **MUST read & sign the [Exceptionless Contribution License Agreement](http://cla-assistant.io/exceptionless/Exceptionless.Net)**. The Exceptionless team is legally prevented from accepting any pull requests from users who have not signed the CLA first.
 
 ## Reporting Bugs