Relative link to local types in builtin types doc

These links work for locally rendered offline documentation.
diff --git a/src/lib.rs b/src/lib.rs
index 74e103a..304a87a 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -308,9 +308,9 @@
 //! <tr><td>String</td><td>rust::String</td><td></td></tr>
 //! <tr><td>&amp;str</td><td>rust::Str</td><td></td></tr>
 //! <tr><td>&amp;[u8]</td><td>rust::Slice&lt;uint8_t&gt;</td><td><sup><i>arbitrary &amp;[T] not implemented yet</i></sup></td></tr>
-//! <tr><td><a href="https://docs.rs/cxx/0.2/cxx/struct.CxxString.html">CxxString</a></td><td>std::string</td><td><sup><i>cannot be passed by value</i></sup></td></tr>
+//! <tr><td><a href="struct.CxxString.html">CxxString</a></td><td>std::string</td><td><sup><i>cannot be passed by value</i></sup></td></tr>
 //! <tr><td>Box&lt;T&gt;</td><td>rust::Box&lt;T&gt;</td><td><sup><i>cannot hold opaque C++ type</i></sup></td></tr>
-//! <tr><td><a href="https://docs.rs/cxx/0.2/cxx/struct.UniquePtr.html">UniquePtr&lt;T&gt;</a></td><td>std::unique_ptr&lt;T&gt;</td><td><sup><i>cannot hold opaque Rust type</i></sup></td></tr>
+//! <tr><td><a href="struct.UniquePtr.html">UniquePtr&lt;T&gt;</a></td><td>std::unique_ptr&lt;T&gt;</td><td><sup><i>cannot hold opaque Rust type</i></sup></td></tr>
 //! <tr><td>fn(T, U) -&gt; V</td><td>rust::Fn&lt;V(T, U)&gt;</td><td><sup><i>only passing from Rust to C++ is implemented so far</i></sup></td></tr>
 //! <tr><td>Result&lt;T&gt;</td><td>throw/catch</td><td><sup><i>allowed as return type only</i></sup></td></tr>
 //! </table>