Pay homage to sequential iteration using "in"

... as suggested [here](https://github.com/JuliaLang/DataStructures.jl/pull/52/files#r16639180).

doc/stdlib/base.rst

@@ -502,7 +502,7 @@ Iteration
 Sequential iteration is implemented by the methods ``start``, ``done``, and
 ``next``. The general ``for`` loop::
 
-    for i = I
+    for i = I   # or  "for i in I"
       # body
     end