Update design_patterns.cpp

You misspelled the word ‘something’.

Author: Car111

cpp/design_patterns.cpp

@@ -15,7 +15,7 @@
     - there is no `Iterator` interface that iterates over anything in the stdlib
         for performance reasons.
 
-        By iterable understand somtehing that has an `::iterator`,
+        By iterable understand something that has an `::iterator`,
         a `begin()` and an `end()` methods, like stl containers
     */
     class VisibleInnerIterable {