Merge pull request #4247 from rust-lang/ch-fix-another-chapter-eighteen

Another Ch. 17 -> 18 fix

Author: chriskrycho

src/ch05-03-method-syntax.md

@@ -5,7 +5,7 @@ name, they can have parameters and a return value, and they contain some code
 that’s run when the method is called from somewhere else. Unlike functions,
 methods are defined within the context of a struct (or an enum or a trait
 object, which we cover in [Chapter 6][enums]<!-- ignore --> and [Chapter
-17][trait-objects]<!-- ignore -->, respectively), and their first parameter is
+18][trait-objects]<!-- ignore -->, respectively), and their first parameter is
 always `self`, which represents the instance of the struct the method is being
 called on.