Updated collections.md: changed julia to jldoctest without modifying code

Author: vivekTiw120303

doc/src/base/collections.md

@@ -5,15 +5,15 @@
 Sequential iteration is implemented by the [`iterate`](@ref) function.
 The general `for` loop:
 
-```julia
-for i in iter   # or  "for i = iter"
-    # body
-end
+```jldoctest
+julia> for i in iter   # or  "for i = iter"
+         # body
+       end
 ```
 
 is translated into:
 
-```julia
+```jldoctest
 next = iterate(iter)
 while next !== nothing
     (i, state) = next