spec: clarify zero value for complex types

spec: clarify zero value for complex types

The enumeration of numeric types missed the complex types.
Clarify by removing the explicit enumeration and referring
to numeric types instead.

Fixes #21579.

Change-Id: If36c2421f8501eeec82a07f442ac2e16a35927ba
Reviewed-on: https://go-review.googlesource.com/58491
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>