langref: neaten up @errSetCast example

doc/langref.html.in

@@ -10267,7 +10267,7 @@ pub fn main() void {
     foo(Set1.B);
 }
 fn foo(set1: Set1) void {
-    const x = @as(Set2, @errSetCast(set1));
+    const x: Set2 = @errSetCast(set1);
     std.debug.print("value: {}\n", .{x});
 }
       {#code_end#}