Fix doc link.

doc/ext_ffi_tutorial.html

@@ -460,7 +460,7 @@ define <tt>__index</tt> and <tt>__newindex</tt> functions instead.
 <span class="mark">&#9316;</span> This associates the metamethods with
 our C&nbsp;type. This only needs to be done once. For convenience, a
 constructor is returned by
-<a href="ffi_ext_api.html#ffi_metatype"><tt>ffi.metatype()</tt></a>.
+<a href="ext_ffi_api.html#ffi_metatype"><tt>ffi.metatype()</tt></a>.
 We're not required to use it, though. The original C&nbsp;type can still
 be used e.g. to create an array of points. The metamethods automatically
 apply to any and all uses of this type.