Fix typos found by codespell

Author: cclauss

CONTRIBUTING.md

@@ -32,7 +32,7 @@ For filing a PR for a new question, please open a issue first for the same and t
 
 ### Bookmarks
 
-The Bookmarks directory is for storing awesome links to articles, videos, slides, talks, etc that are interesting and can help in increasing knowledge for a specific topic. There has been many PRs for adding new links in Bookmarks and most of them refered to either a whole book or DS Algo MOOCs, please refrain from adding them. Here are some points that you must consider before contributing to Bookmarks -
+The Bookmarks directory is for storing awesome links to articles, videos, slides, talks, etc that are interesting and can help in increasing knowledge for a specific topic. There has been many PRs for adding new links in Bookmarks and most of them referred to either a whole book or DS Algo MOOCs, please refrain from adding them. Here are some points that you must consider before contributing to Bookmarks -
 
 * Don't add a link to a MOOC or course (like DS Algo MIT lectures)
 

README.md

@@ -2,7 +2,7 @@
 
 # Python Data Structures and Algorithms
 
-This repository contains data structures and algorithms concepts and questions userful for interviews in Python.
+This repository contains data structures and algorithms concepts and questions useful for interviews in Python.
 
 ## :dart: Objective
 

notes.md

@@ -19,7 +19,7 @@
 2. Dynamic memory allocation [Why?](https://www.slideshare.net/profansari/linked-list-static-and-dynamic-memory-allocation)
 3. Maintaining directory of names
 4. Image viewer, where images are linked in slideshows
-5. Previos and next page in web browsers
+5. Previous and next page in web browsers
 6. Playlist in music players
 
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@@ -85,7 +85,7 @@
 
 ### Advantages of BST over heap
 
-1. Search for arbitary element is O(log(n)) in BST whereas it is O(n) for heap. [Reference](https://stackoverflow.com/questions/6147242/heap-vs-binary-search-tree-bst)
+1. Search for arbitrary element is O(log(n)) in BST whereas it is O(n) for heap. [Reference](https://stackoverflow.com/questions/6147242/heap-vs-binary-search-tree-bst)
 
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