Update contributors.

book/src/contributors.md

@@ -13,7 +13,7 @@
 The following people have contributed to this project:
 
 * Herbert Wolverson, the primary author.
-* [Marius Gedminas](https://github.com/mgedmin) provided some fixes to the visibility system, and chapter 5. Corrected a typo in chapter 7, fixed a Markdown error in chapter 7, and fixed indentation in chapter 13.
+* [Marius Gedminas](https://github.com/mgedmin) provided some fixes to the visibility system, and chapter 5. Corrected a typo in chapter 7, fixed a Markdown error in chapter 7, fixed indentation in chapter 13, and a typo in chapter 18.
 
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