go/token: remove excess parenthesis in NoPos.IsValid() documentation

Fixes #20294

Change-Id: I32ac862fe00180210a04103cc94c4d9fef5d1b6c
Reviewed-on: https://go-review.googlesource.com/42992
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: ibrasho

src/go/token/position.go

@@ -71,7 +71,7 @@ func (pos Position) String() string {
 type Pos int
 
 // The zero value for Pos is NoPos; there is no file and line information
-// associated with it, and NoPos().IsValid() is false. NoPos is always
+// associated with it, and NoPos.IsValid() is false. NoPos is always
 // smaller than any other Pos value. The corresponding Position value
 // for NoPos is the zero value for Position.
 //