Fix some warnings

[ameknite]

Objective

• Fix these warnings

warning: unused doc comment
  --> /bevy/crates/bevy_pbr/src/light.rs:62:13
   |
62 |             /// Luminous power in lumens
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
63 |             intensity: 800.0, // Roughly a 60W non-halogen incandescent bulb
   |             ---------------- rustdoc does not generate documentation for expression fields
   |
   = help: use `//` for a plain comment
   = note: `#[warn(unused_doc_comments)]` on by default

warning: `&` without an explicit lifetime name cannot be used here
  --> /bevy/crates/bevy_asset/src/lib.rs:89:32
   |
89 |     const DEFAULT_FILE_SOURCE: &str = "assets";
   |                                ^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #115010 <https://github.com/rust-lang/rust/issues/115010>
   = note: `#[warn(elided_lifetimes_in_associated_constant)]` on by default
help: use the `'static` lifetime
   |
89 |     const DEFAULT_FILE_SOURCE: &'static str = "assets";
   |