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/132284Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of May 9, 2018",
+	"Subtitle": "Version of August 30, 2018",
 	"Path": "/ref/spec"
 }-->
 
@@ -2112,7 +2112,7 @@ with initializer expressions but no types:
 i, j := 0, 10
 f := func() int { return 7 }
 ch := make(chan int)
-r, w := os.Pipe(fd)  // os.Pipe() returns two values
+r, w, _ := os.Pipe()  // os.Pipe() returns a connected pair of Files and an error, if any
 _, y, _ := coord(p)   // coord() returns three values; only interested in y coordinate
 </pre>