Fix broken link to contexts guide (#3925)
Fix broken link to contexts guide (#3925)

The contexts guide link and name is pluralized where this page was linking to a singular version that doesn't exist.
0x00evil/phoenix,michalmuskala/phoenix,phoenixframework/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,rafbgarcia/phoenix,0x00evil/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,optikfluffel/phoenix,nshafer/phoenix,nshafer/phoenix,optikfluffel/phoenix,eksperimental/phoenix,eksperimental/phoenix,michalmuskala/phoenix,0x00evil/phoenix,nshafer/phoenix