spec: fix prose about terminating statements

CL 85215 added prose to provide some minimal intuition for the
definition of a "terminating statement". While the original definition
was perfectly fine, the added prose was actually incorrect: If the
terminating statement is a goto, it might jump to a labeled statement
following that goto in the same block (it could be the very next
statement), and thus a terminating statement does not in fact
"prevent execution of all statements that lexically appear after
it in the same block".

Rather than explaining the special case for gotos with targets that
are lexically following the goto in the same block, this CL opts for
a simpler approach.

Thanks to @3bodar (Github) for finding this.

Fixes #48323.

Change-Id: I8031346250341d038938a1ce6a75d3e687d32c37
Reviewed-on: https://go-review.googlesource.com/c/go/+/349172


Trust: Robert Griesemer <gri@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>