Update ISLE error message: unknown term in expr, not pattern. (#4775)

This was likely a copy-paste from the `ast::Pattern` case, but here it
is checking a term name in `ast::Expr` and so should say "... in
expression", not "... in pattern".

cranelift/isle/isle/isle_examples/fail/extra_parens.isle

@@ -0,0 +1,6 @@
+(type u32 (primitive u32))
+
+(decl f (u32) u32)
+;; Should get an error about `x` not being a term, with a suggestion that it is
+;; a bound var instead.
+(rule (f x) (x))