cmd/compile: fix error message wording

cmd/compile: fix error message wording

CL 450136 fixed a different copy of this error but missed this one.

With the compiler fix from CL 451555 rolled back to produce the error,
this is the text before this CL:

	b.go:9:15: internal compiler error: 'init': Value live at entry. It shouldn't be. func init, node a.i, value nil

And this CL changes it to:

	b.go:9:15: internal compiler error: 'init': value a.i (nil) incorrectly live at entry

matching the same change in the earlier CL.

Change-Id: I33e6b91477e1a213a6918c3ebdea81273be7d235
Reviewed-on: https://go-review.googlesource.com/c/go/+/452816
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>