Fix minor doc typo (#13776)

Don't think any description is needed, just happened to stumble on this
while reading up on how to use `Stopwatch` :)

crates/bevy_time/src/stopwatch.rs

@@ -2,7 +2,7 @@
 use bevy_reflect::{prelude::*, Reflect};
 use bevy_utils::Duration;
 
-/// A Stopwatch is a struct that track elapsed time when started.
+/// A Stopwatch is a struct that tracks elapsed time when started.
 ///
 /// Note that in order to advance the stopwatch [`tick`](Stopwatch::tick) **MUST** be called.
 /// # Examples