docs: Fix build error in book chapter 1

Seems to be the same code as the hello_world example, so I just copied the semi-colon from there.

book/src/chapter_1.md

@@ -35,7 +35,7 @@ fn startup(
                 }}
             />
         </KayakAppBundle>
-    }
+    };
     
     commands.spawn(UICameraBundle::new(widget_context));
 }