Fix typo Flash Card -> flashcard (donnemartin#168)

Author: donnemartin

README.md

@@ -5,7 +5,7 @@
 
 ## April 2017 - Huge Update!
 
-[**Overhauled**](https://github.com/donnemartin/interactive-coding-challenges/pull/153) to now include **120 challenges and solutions** and added [Anki Flash Cards](#anki-flashcards-coding-and-design).
+[**Overhauled**](https://github.com/donnemartin/interactive-coding-challenges/pull/153) to now include **120 challenges and solutions** and added [Anki flashcards](#anki-flashcards-coding-and-design).
 
 Also included are **unit tested reference implementations** of various [data structures](#reference-implementations-data-structures) and [algorithms](#reference-implementations-algorithms).