doc/src/manual/arrays.md: fix typo (#52502)

Mix-up between `I_k` and `i_k`. I bolded the wrong part.

Author: jcsahnwaldt

doc/src/manual/arrays.md

@@ -603,7 +603,7 @@ overwritten with the value of `X`, [`convert`](@ref)ing to the
 If any index `I_k` is itself an array, then the right hand side `X` must also be an
 array with the same shape as the result of indexing `A[I_1, I_2, ..., I_n]` or a vector with
 the same number of elements. The value in location `I_1[i_1], I_2[i_2], ..., I_n[i_n]` of
-`A` is overwritten with the value `X[I_1, I_2, ..., I_n]`, converting if necessary. The
+`A` is overwritten with the value `X[i_1, i_2, ..., i_n]`, converting if necessary. The
 element-wise assignment operator `.=` may be used to [broadcast](@ref Broadcasting) `X`
 across the selected locations: