Fix typo in copy! implementation

Author: timholy

blog/_posts/2016-03-27-arrays-iteration.md

@@ -321,7 +321,7 @@ with different iterator types.  We're tempted to write this as
 ```jl
 function copy!(dest, src)
     for (idest, isrc) in zip(eachindex(dest), eachindex(src))
-        dest[idest] = src[idest]
+        dest[idest] = src[isrc]
     end
     dest
 end