remove an @extref

Author: ablaom

src/testapi.jl

@@ -48,7 +48,7 @@ The following are *not* tested:
 - The veracity of `LearnAPI.is_pure_julia(learner)`.
 
 - The second of the two contracts appearing in the
-  [`LearnAPI.target_observation_scitype`](@extref) docstring. The first contract is only
+  [`LearnAPI.target_observation_scitype`](@ref) docstring. The first contract is only
   tested if `LearnAPI.data_interface(learner)` is `LearnAPI.RandomAccess()` or
   `LearnAPI.FiniteIterable()`.