Typo fix in botorch overview

Summary: Fixes a minor typo in the overview doc

Reviewed By: danielrjiang

Differential Revision: D15154439

fbshipit-source-id: d9d3a2daba16d67c8f245c97d6c7c73b1bb4ea31

Author: bletham

docs/overview.md

@@ -115,7 +115,7 @@ an `Objective` module, which returns a one-dimensional output that is passed to
 the acquisition function. The `MCAcquisitionFunction` class defaults its
 objective to `IdentityMCObjective`, which simply returns the last dimension of
 the model output. Thus, for the standard use case of a single-output GP that
-directly models the black box function $f$, no special objective required.
+directly models the black box function $f$, no special objective is required.
 For more details on the advanced features enabled by the `Objective`, see
 [Objectives](objectives).