Nit: Definitions of SetMutableBinding methods refer to method by wrong name

[Jim Blandy]

The definitions of the SetMutableBinding method for both declarative and 
object environment records (10.2.1.1.3 and 10.2.1.2.2) refer to 
"SetMutableValue" in their first sentence.  This name doesn't occur 
anywhere else; I assume they're supposed to be SetMutableBinding.