langref: fix the slice_bounds.zig doctest

In the slice_bounds.zig doctest, the code "const slice = array[2..4]" is
incorrect, since the actual type is a pointer to an array, instead of a
slice.

Use a runtime know value to slice the array.

doc/langref.html.in

@@ -2691,7 +2691,8 @@ const expect = @import("std").testing.expect;
 
 test "pointer slicing" {
     var array = [_]u8{ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
-    const slice = array[2..4];
+    var start: usize = 2;
+    const slice = array[start..4];
     try expect(slice.len == 2);
 
     try expect(array[3] == 4);