docs: fix recursive example in template_syntax.md

[m4tx]

The original description mentions askama issues which is confusing, since they don't exist in rinja repo and, most importantly, the example doesn't compile at all. This fixes both issues.

Output

Tested with the following code:

use rinja::Template;

#[derive(Template)]
#[template(source = r#"
{{ name }} {
{% for item in children %}
   {{ item.render().unwrap() }}
{% endfor %}
}
"#, ext = "html", escape = "none")]
struct Item<'a> {
    name: &'a str,
    children: &'a [Item<'a>],
}

fn main() {
    let xd = Item {
        name: "test1",
        children: &[
            Item {
                name: "test2",
                children: &[
                    Item {
                        name: "test3",
                        children: &[]
                    }
                ]
            }
        ]
    };

    println!("{}", xd.render().unwrap());
}

Which prints:

test1 {

   
test2 {

   
test3 {

}


}


}

Old behavior

The old code fails to compile with:

error: reached the recursion limit while instantiating `filters::escape::_::<impl FastWritable for &Item<'_>>::write_into::<filters::escape::EscapeWriter<&mut filters::escape::EscapeWriter<..., ...>, ...>>`
   --> /home/m4tx/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rinja-0.3.5/src/filters/escape.rs:511:17
    |
511 |                 <T>::write_into(self, dest)
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
note: `filters::escape::_::<impl FastWritable for &T>::write_into` defined here
   --> /home/m4tx/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rinja-0.3.5/src/filters/escape.rs:510:13
    |
510 |             fn write_into<W: fmt::Write + ?Sized>(&self, dest: &mut W) -> fmt::Result {
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: the full type name has been written to '/home/m4tx/projects/rust-test/target/debug/deps/rust_test-7872518c4b30a6ab.long-type.txt'

[Kijewski]

Thank you!