Fix newline in PointLightShadowMap comment

[greeble-dev]

A clippy failure slipped into #18768, although I'm not sure why CI didn't catch it.

> cargo clippy --version
clippy 0.1.85 (4eb161250e 2025-03-15)

> cargo run -p ci
...
error: empty line after doc comment
   --> crates\bevy_pbr\src\light\mod.rs:105:5
    |
105 | /     /// The width and height of each of the 6 faces of the cubemap.
106 | |
    | |_^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#empty_line_after_doc_comments
    = note: `-D clippy::empty-line-after-doc-comments` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::empty_line_after_doc_comments)]`
    = help: if the empty line is unintentional remove it
help: if the documentation should include the empty line include it in the comment
    |
106 |     ///
    |

[greeble-dev]

Can't repro on 0.1.86, which is what CI is using. Does repro on 0.1.85.