fix Introspection link (graphql#147)

Author: poopoothegorilla

site/learn/Learn-Queries.md

@@ -283,5 +283,5 @@ Given that there are some situations where you don't know what type you'll get b
 
 In the above query, `search` returns a union type that can be one of three options. It would be impossible to tell apart the different types from the client without the `__typename` field.
 
-GraphQL services provide a few meta fields, the rest of which are used to expose the [Introspection](./introspection/) system.
+GraphQL services provide a few meta fields, the rest of which are used to expose the [Introspection](../introspection/) system.