Fix error in docstring of ConstrainedExpectedImprovement (#1499)

Summary: Pull Request resolved: #1499

Reviewed By: esantorella

Differential Revision: D41249851

Pulled By: Balandat

fbshipit-source-id: 86cea4514778872d9adf2e100f7d543ea261c8f7

Author: Balandat

botorch/acquisition/analytic.py

@@ -373,7 +373,7 @@ def __init__(
         r"""Analytic Constrained Expected Improvement.
 
         Args:
-            model: A fitted single-outcome model.
+            model: A fitted multi-output model.
             best_f: Either a scalar or a `b`-dim Tensor (batch mode) representing
                 the best feasible function value observed so far (assumed noiseless).
             objective_index: The index of the objective.