fix typo in replace_storage docs (#176)

Author: Pietro Vertechi

src/utils.jl

@@ -134,7 +134,7 @@ end
 """
     StructArrays.replace_storage(f, s::StructArray)
 
-Change storage type for components: each array `v` is replaced by `f(v)`. `f(v) is expected to have the same
+Change storage type for components: each array `v` is replaced by `f(v)`. `f(v)` is expected to have the same
 `eltype` and `size` as `v`.
 
 ## Examples