Fix doc_markdown lints in bevy_gltf (#3474)

#3457 adds the `doc_markdown` clippy lint, which checks doc comments to make sure code identifiers are escaped with backticks. This causes a lot of lint errors, so this is one of a number of PR's that will fix those lint errors one crate at a time.

This PR fixes lints in the `bevy_gltf` crate.

crates/bevy_gltf/src/loader.rs

@@ -653,7 +653,7 @@ fn texture_address_mode(gltf_address_mode: &gltf::texture::WrappingMode) -> Addr
     }
 }
 
-/// Maps the primitive_topology form glTF to wgpu.
+/// Maps the `primitive_topology` form glTF to `wgpu`.
 fn get_primitive_topology(mode: Mode) -> Result<PrimitiveTopology, GltfError> {
     match mode {
         Mode::Points => Ok(PrimitiveTopology::PointList),