[plt-scheme] slightly misleading error message

I was checking whether mutation of parameters is forbidden in Advanced 
Student (which it should be, because the semantics don't explain it) and 
my attempt to do so did result in an error message, but the message was:

set!: expected a defined name after `set!', but found a function 
argument name

with the first "n" in (set! n (add1 n)) highlighted. This is in v360. --PR