Remove some UB from examples in performance tips (#54221)

Remove some UB from examples in performance tips

Fixes #54218

I also changed the iteration specification to better align with
elsewhere in the document.

A separate PR may want to delete or revise this section if automatic
simd has improved. Nevertheless, I think this PR should be
unobjectionable as is.

Author: LilithHafner

doc/src/manual/performance-tips.md

@@ -1446,15 +1446,15 @@ the optimizer from trying to be too clever and defeat our benchmark):
 ```julia
 @noinline function inner(x, y)
     s = zero(eltype(x))
-    for i=eachindex(x)
+    for i in eachindex(x, y)
         @inbounds s += x[i]*y[i]
     end
     return s
 end
 
 @noinline function innersimd(x, y)
     s = zero(eltype(x))
-    @simd for i = eachindex(x)
+    @simd for i in eachindex(x, y)
         @inbounds s += x[i] * y[i]
     end
     return s