erlang.mk: Update repository URL for proper dep

Proper was moved under the `proper-testing` org.

Author: the-mikedavis

erlang.mk

@@ -2680,7 +2680,7 @@ pkg_proper_name = proper
 pkg_proper_description = PropEr: a QuickCheck-inspired property-based testing tool for Erlang.
 pkg_proper_homepage = http://proper.softlab.ntua.gr
 pkg_proper_fetch = git
-pkg_proper_repo = https://github.com/manopapad/proper
+pkg_proper_repo = https://github.com/proper-testing/proper
 pkg_proper_commit = master
 
 PACKAGES += props