updated book links

Author: learnbyexample

README.md

@@ -18,9 +18,9 @@ See [Version_changes.md](./Version_changes.md) to keep track of changes made to
 
 You can purchase the book using these links:
 
-* https://gumroad.com/l/js_regexp
+* https://learnbyexample.gumroad.com/l/js_regexp
 * https://leanpub.com/js_regexp
-* You can also get the book as part of **Awesome Regex** bundle from https://gumroad.com/l/regex or https://leanpub.com/b/regex
+* You can also get the book as part of **Awesome Regex** bundle from https://learnbyexample.gumroad.com/l/regex or https://leanpub.com/b/regex
 * See https://learnbyexample.github.io/books/ for list of other books
 
 For a preview of the book, see [sample chapters](https://github.com/learnbyexample/learn_js_regexp/blob/master/sample_chapters/js_regexp_sample.pdf)