trpl: fix some broken internal links

src/doc/trpl/enums.md

@@ -66,3 +66,4 @@ equality yet, but we’ll find out in the [`traits`][traits] section.
 
 [match]: match.html
 [if-let]: if-let.html
+[traits]: traits.html

src/doc/trpl/match.md

@@ -97,4 +97,4 @@ Unlike the previous uses of `match`, you can’t use the normal `if`
 statement to do this. You can use the [`if let`][if-let] statement,
 which can be seen as an abbreviated form of `match`.
 
-[if-let][if-let.html]
+[if-let]: if-let.html

src/doc/trpl/structs.md

@@ -196,3 +196,5 @@ useful. For instance, a library may ask you to create a structure that
 implements a certain [trait][trait] to handle events. If you don’t have
 any data you need to store in the structure, you can just create a
 unit-like struct.
+
+[trait]: traits.html