Add missing Collections article to index

Author: seancorfield

doc/cljdoc.edn

@@ -1,6 +1,7 @@
 {:cljdoc.doc/tree [["Readme" {:file "README.md"}]
                    ["Changes" {:file "CHANGELOG.md"}]
                    ["Getting Started" {:file "doc/getting-started.md"}]
+                   ["Collections" {:file "doc/collections.md"}]
                    ["Useful Predicates" {:file "doc/useful-predicates.md"}]
                    ["Expecting More" {:file "doc/more.md"}]
                    ["Expecting Side Effects" {:file "doc/side-effects.md"}]