README: Change 'OBS' to 'Note' (ossu#454)

As per issue ossu#453

Author: Florian Hoornaar

README.md

@@ -166,7 +166,7 @@ Courses | Duration | Effort | Prerequisites
 
 #### Readings
 - **Required** to learn about monads, laziness, purity: [Learn You a Haskell for a Great Good!](http://learnyouahaskell.com/)
-  - **OBS**: probably the best resource to learn Haskell: [Haskell Programming from First Principles](http://haskellbook.com/) `paid`
+  - **Note**: probably the best resource to learn Haskell: [Haskell Programming from First Principles](http://haskellbook.com/) `paid`
 - **Required**, to learn about logic programming, backtracking, unification: [Learn Prolog Now!](http://www.learnprolognow.org/)
 
 ### Core math