Update link to user's guide documentation

Author: Sunderlandkyl

usersguide.md

@@ -4,7 +4,7 @@ title: User's guide
 group: navigation
 order: 4
 description: ""
-linkurl: "http://perk-software.cs.queensu.ca/plus/doc/nightly/user/index.html"
+linkurl: "https://pluslib.readthedocs.io/en/latest/"
 linkname: "Plus user's guide"
 ---
 {% include JB/setup %}