[customization.point.object] fix grammar (#4950)

Author: jwakely

source/lib-intro.tex

@@ -813,7 +813,7 @@
 customization point objects specify that lookup for these expressions is
 performed in a context that includes deleted overloads matching the signatures
 of overloads defined in namespace \tcode{std}. When the deleted overloads are
-viable, program-defined overloads need be more specialized\iref{temp.func.order}
+viable, program-defined overloads need to be more specialized\iref{temp.func.order}
 or more constrained\iref{temp.constr.order} to be used by a customization point
 object.
 \end{note}