Fix 'Dev' link in compatibility table (#2008)

Author: yvonnefroehlich

README.rst

@@ -233,7 +233,7 @@ Compatibility with GMT/Python/NumPy versions
       - GMT
       - Python
       - NumPy
-    * - `Dev <https://github.com/GenericMappingTools/pygmt/milestone/9>`_ (upcoming release)
+    * - `Dev <https://github.com/GenericMappingTools/pygmt/milestones>`_ (upcoming release)
       - `Dev Documentation <https://www.pygmt.org/dev>`_ (reflects `main branch <https://github.com/GenericMappingTools/pygmt>`_)
       - >=6.3.0
       - >=3.8