[fix] doc typo in lib/stdlib/core/list.opa
[Fix] doc error

- * [a <+> b] is the same thing as <>{a}{b}</>
+ * [a <+> b] is the same thing as [<>{a}{b}</>]
Merge pull request #97 from TuTu/patch-1

[contrib] List: doc typo in lib/stdlib/core/list.opa