Update import in if_else_while.md example

This example doesn't compile with nim-0.15. Importing *random* instead of *math* made the trick for me.

content/content/if_else_while.md

@@ -11,7 +11,7 @@ When inside a loop, `continue` can be used to skip the rest of the loop body and
 Along with [its other uses](/block/), the `block` statement can be used to create a label so that it's possible to break out of nested loops.
 
 <!--skip-->``` nim
-import strutils, math
+import strutils, random
 
 randomize()
 let answer = random(10) + 1