Update README.md

Co-authored-by: Andreas Mueller <t3kcit@gmail.com>

Author: Cyebukayire

README.md

@@ -6,7 +6,7 @@ This repository holds the code for the forthcoming book "Introduction to Machine
 Learning with Python" by [Andreas Mueller](http://amueller.io) and [Sarah Guido](https://twitter.com/sarah_guido).
 You can find details about the book on the [O'Reilly website](http://shop.oreilly.com/product/0636920030515.do).
 
-The books require the current stable version of scikit-learn, that is
+The book requires the current stable version of scikit-learn, that is
 0.20.0.  Most of the book can also be used with previous versions of
 scikit-learn, though you need to adjust the import for everything from the
 ``model_selection`` module, mostly ``cross_val_score``, ``train_test_split``