doc: fix os.Pipe() call in the example.

Short variable declarations example passes an fd argument to os.Pipe call.
However, os.Pipe() takes no arguments and returns 2 Files and an error:
https://golang.org/src/os/pipe_linux.go?s=319:360#L1

Fixes: #27384

Change-Id: I0a709f51e0878c57185d901b899d209f001dfcce
Reviewed-on: https://go-review.googlesource.com/132284


Reviewed-by: Robert Griesemer <gri@golang.org>