Merge pull request #2867 from jruderman/commadp

Chapter 17.1: add missing comma

src/ch17-01-what-is-oo.md

@@ -12,7 +12,7 @@ Rust supports it.
 
 The book *Design Patterns: Elements of Reusable Object-Oriented Software* by
 Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Addison-Wesley
-Professional, 1994) colloquially referred to as *The Gang of Four* book, is a
+Professional, 1994), colloquially referred to as *The Gang of Four* book, is a
 catalog of object-oriented design patterns. It defines OOP this way:
 
 > Object-oriented programs are made up of objects. An *object* packages both