Updated collections.md from julia to jldoctest

Author: vivekTiw120303

doc/src/base/collections.md

@@ -5,21 +5,32 @@
 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 1:3   # or  "for i = iter"
+         println(i)
+       end
+1
+2
+3
 ```
 
 is translated into:
 
 ```julia
-next = iterate(iter)
-while next !== nothing
-    (i, state) = next
-    # body
-    next = iterate(iter, state)
-end
+julia> iter = 1:3
+1:3
+
+julia> next = iterate(iter)
+(1,1)
+
+julia> while next !== nothing
+           (i, state) = next
+           println(i)
+           next = iterate(iter, state)
+       end
+1
+2
+3
 ```
 
 The `state` object may be anything, and should be chosen appropriately for each iterable type.