Fix a typo on aliasing

Author: JohnTitor

src/aliasing.md

@@ -88,7 +88,7 @@ These optimizations also tend to prove the soundness of bigger optimizations
 such as loop vectorization, constant propagation, and dead code elimination.
 
 In the previous example, we used the fact that `&mut u32` can't be aliased to prove
-that writes to `*output` can't possibly affect `*input`. This let us cache `*input`
+that writes to `*output` can't possibly affect `*input`. This lets us cache `*input`
 in a register, eliminating a read.
 
 By caching this read, we knew that the write in the `> 10` branch couldn't