syscall: update doc comment to match behavior for Proc.Call

golang.org/cl/147117 increased the number of arguments permitted
by Proc.Call on Windows, but the doc comment was never updated.

Change-Id: Iea5eb9e0aafbc1025d5fcb8665d028b2254c183a
Reviewed-on: https://go-review.googlesource.com/c/153825Reviewed-by: Channing Kimble-Brown <ckimblebrown@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/syscall/dll_windows.go

@@ -132,7 +132,7 @@ func (p *Proc) Addr() uintptr {
 
 //go:uintptrescapes
 
-// Call executes procedure p with arguments a. It will panic, if more than 15 arguments
+// Call executes procedure p with arguments a. It will panic if more than 18 arguments
 // are supplied.
 //
 // The returned error is always non-nil, constructed from the result of GetLastError.