contributing: Link Open Collective contribute page (#2939)

Given how Open Collective page is organized, the page with clear contributing options is contribute which has all the options and nothing else, not the home (index) page for a collective. GitHub support suggests to change the configuration by adding /contribute to make the change.

.github/FUNDING.yml

@@ -1,2 +1,2 @@
 ---
-open_collective: grass
+open_collective: grass/contribute