Fix #39367, doc for inbounds use each index (#39369)

(cherry picked from commit f31ef76)

Author: mbauman

base/essentials.jl

@@ -538,7 +538,7 @@ element `i` of array `A` is skipped to improve performance.
 ```julia
 function sum(A::AbstractArray)
     r = zero(eltype(A))
-    for i = 1:length(A)
+    for i in eachindex(A)
         @inbounds r += A[i]
     end
     return r