go/ast/inspector: update outdated documentation

CL 666056 removed Cursor.Stack, but the documentation for Inspector.Root
still refers to it.

Change-Id: I345ddbbbe270f5ac96d38bbde2b177a297bf2f73
Reviewed-on: https://go-review.googlesource.com/c/tools/+/697255
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: DanielMorsing

go/ast/inspector/cursor.go

@@ -40,7 +40,7 @@ type Cursor struct {
 // Root returns a cursor for the virtual root node,
 // whose children are the files provided to [New].
 //
-// Its [Cursor.Node] and [Cursor.Stack] methods return nil.
+// Its [Cursor.Node] method return nil.
 func (in *Inspector) Root() Cursor {
 	return Cursor{in, -1}
 }