Fix Archlinux Installation Instructions
Fix Archlinux Installation Instructions

Suggesting -Sy is a terrible idea[1]. This commit replaces it with -S
which is the default on how packages should be added on Archlinux.

Resolves #662.

[1]: https://bbs.archlinux.org/viewtopic.php?id=89328

crownedgrouse/erlang.mk,hairyhum/erlang.mk,rabbitmq/erlang.mk,jj1bdx/erlang.mk,ninenines/erlang.mk,nevar/erlang.mk,ingwinlu/erlang.mk