Rectify return type hints in logprob module rewrites

pymc/logprob/tensor.py

@@ -199,7 +199,7 @@ def logprob_join(op, values, axis, *base_rvs, **kwargs):
 @node_rewriter([MakeVector, Join])
 def find_measurable_stacks(
     fgraph, node
-) -> Optional[List[TensorVariable, TensorVariable]]:
+) -> Optional[List[TensorVariable]]:
     r"""Finds `Joins`\s and `MakeVector`\s for which a `logprob` can be computed."""
 
     rv_map_feature: Optional[PreserveRVMappings] = getattr(fgraph, "preserve_rv_mappings", None)