Fixed minor link error in docs (#7572)

# Objective
Fix #7571
## Solution
* Removed the offending line. 
***
## Changelog
* Removed
* * The line: ``\\ [`apply_system_buffers`]: bevy_ecs::prelude::apply_system_buffers`` from `bevy_app` crate, which overrides the link in that specific comment block.

Co-authored-by: lupan <kallll5@hotmail.com>

crates/bevy_app/src/lib.rs

@@ -165,8 +165,6 @@ impl CoreSet {
 /// The `*Flush` sets are assigned to the copy of [`apply_system_buffers`]
 /// that runs immediately after the matching system set.
 /// These can be useful for ordering, but you almost never want to add your systems to these sets.
-///
-/// [`apply_system_buffers`]: bevy_ecs::prelude::apply_system_buffers
 #[derive(Debug, Hash, PartialEq, Eq, Clone, SystemSet)]
 pub enum StartupSet {
     /// Runs once before [`StartupSet::Startup`].