Move sentence in Classes.md
Move sentence in Classes.md

The text did not make sense with the sentence about calling super() between a sentence that leads into an example (with a colon) and the example. 

This issue was introduced in this commit: https://github.com/Microsoft/TypeScript-Handbook/pull/81/commits/d0fa4643d39af583045da6752c4dab34812efcc3
zspitz/TypeScript-Handbook,Microsoft/TypeScript-Handbook,gooddaytoday/TypeScript-Handbook-RU