format(README): change header types

Author: jessicayung

README.md

@@ -9,13 +9,13 @@ Notes and code from HackerRank Challenges and other programming problems.
 2. Programming Problems: `programming-problems`
 3. Flashcards: `flashcards`
 
-### 1. Algorithm implementations
+## 1. Algorithm implementations
 
 Implementations of key algorithms for tackling programming problems. Language: Python 3.
 
-### 2. Programming Problems
+## 2. Programming Problems
 
-#### Problems by Problem Areas
+### Problems by Problem Areas
 Problems classified by area are from HackerRank. 
 
 **Finding these problems**: E.g. problems in the category 'Mathematics' are in a directory called 'mathematics'.
@@ -28,12 +28,12 @@ Problems classified by area are from HackerRank.
 * Statistics 
   * HackerRank's Intro to Statistics Series
 
-#### Problem Sources (non-HackerRank)
+### Problem Sources (non-HackerRank)
 These problems are not classified by area, but by source. These problems are not from HackerRank.
 * InterviewCake
 * Cracking the Coding Interview by Gabrielle Laakmann-McDowell (`laakmann-mcdowell`)
 
-### 3. Flashcards
+## 3. Flashcards
 Flashcard content to import into Anki (flashcard software).
 
 * General programming terminology