fix typos in github urls

docs/contributing.md

@@ -2,9 +2,9 @@
 
 This is Open Source software, so [given enough eyeballs, all bugs are shallow](https://en.wikipedia.org/wiki/Linus%27s_Law). Your contributions are more than welcome.
 
-This project is hosted on [GitHub](https://githhub.com/christophevg/bpmn-tools) and these (common) rules apply:
+This project is hosted on [GitHub](https://github.com/christophevg/bpmn-tools) and these (common) rules apply:
 
-* Do use the [issues tracker](https://githhub.com/christophevg/bpmn-tools/issues).
+* Do use the [issues tracker](https://github.com/christophevg/bpmn-tools/issues).
 * Let's discuss any proposed change or fix in an issue, so your work is not done in vain - I hate to reject pull requests...
-* Create [pull requests](https://githhub.com/christophevg/bpmn-tools/pulls) against `next` branch.
+* Create [pull requests](https://github.com/christophevg/bpmn-tools/pulls) against `next` branch.
 * Try to keep pull requests "atomic", and if possible related to an issue.