Fix documentation comments for Rust 1.66. (#3310)

Somewhere after 1.64 but by 1.66, `rustdoc` started checking for
unmatched HTML tags in doc strings, meaning that text like

    /// Convenience function turning Option<Selector> into this enum.

causes problems, since `rustdoc` will pass through `<Selector>` as
HTML tag, which browsers displaying the output will misunderstand.

wgpu-core/src/track/texture.rs

@@ -814,7 +814,7 @@ enum TextureStateProvider<'a> {
  TextureSet { set: &'a TextureStateSet },
 }
 impl<'a> TextureStateProvider<'a> {
- /// Convenience function turning Option<Selector> into this enum.
+ /// Convenience function turning `Option<Selector>` into this enum.
  fn from_option(selector: Option<TextureSelector>, state: TextureUses) -> Self {
  match selector {
  Some(selector) => Self::Selector { selector, state },