Make two docs links relative

Fixes #131.

Author: carols10cents

Diff for: src/ch05-01-method-syntax.md

@@ -179,7 +179,7 @@ Because [`push_str()`] has the following signature:
 fn push_str(&mut self, string: &str) {
 ```
 
-[`push_str()`]: http://doc.rust-lang.org/collections/string/struct.String.html#method.push_str
+[`push_str()`]: ../collections/string/struct.String.html#method.push_str
 
 This automatic referencing behavior works because methods have a clear receiver
 — the type of `self` — and in most cases it’s clear given the receiver and name

Diff for: src/ch06-00-enums.md

@@ -116,7 +116,7 @@ enum IpAddr {
 }
 ```
 
-[IpAddr]: http://doc.rust-lang.org/std/net/enum.IpAddr.html
+[IpAddr]: ../std/net/enum.IpAddr.html
 
 Here’s an enum with a variety of types embedded in its variants: