Change license of book to Creative Commons Attribution-Sharealike

Author: Blake-Madden

docs/manual/images/CC_BY-NC-ND.png

@@ -16,9 +16,9 @@ if (!webshot::is_phantomjs_installed())
 
 This book is the guide to using the *TinyExpr++* library, a C++ formula parsing and evaluation system.
 
-![](manual/images/CC_BY-NC-ND.png)
+![](manual/images/by-sa.png)
 
-This book is licensed under the [Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License](https://creativecommons.org/licenses/by-nc-nd/4.0/).
+This book is licensed under the [Creative Commons Attribution-Sharealike 4.0 International License](https://creativecommons.org/licenses/by-sa/4.0/).
 :::
 
 # Overview {-}