[doc] Fix typo in url

(cherry picked from commit b5db5759)

Signed-off-by: Steffen Müthing <muething@dune-project.org>

CONTRIBUTING.md

@@ -54,7 +54,7 @@ Getting the changes to us
 -------------------------
 
 You should get your changes to us in the following way:
-* Get an account for [our GitLab instance](htpp://gitlab.dune-project.org).
+* Get an account for [our GitLab instance](http://gitlab.dune-project.org).
 * Fork the core module that you want to contribute to, just
   as you would do on GitHub.
 * Push your changes to your fork on some branch.
@@ -67,4 +67,4 @@ You should get your changes to us in the following way:
 
 If you have any questions or complaints about this workflow of
 contributing to Dune, please rant on the
-[dune-devel mailing list](mailto:dune-devel@dune-project.org).
+[dune-devel mailing list](mailto:dune-devel@dune-project.org).
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