fix broken markdown local links in 2018/README.md

Ran `make gen_year_index` to perform the above.

2018/README.md

@@ -50,7 +50,7 @@ It is something completely different.
 The [Most likely to be awarded - 2018/ciura](ciura/index.html) has an amazing vocabulary!
 
 There are also nods to entries of the years [2000/bellard](../2000/bellard/index.html),
-[1989/yang](../1989/yang/index.html), ...
+[2015/yang](../2015/yang/index.html), ...
 
 ...We'll stop spouting spoilers now. Have fun exploring all the entries!
 

2018/index.html

@@ -416,7 +416,7 @@ of the <a href="poikola/index.html">Most stellar - 2018/poikola</a>.</p>
 It is something completely different.</p>
 <p>The <a href="ciura/index.html">Most likely to be awarded - 2018/ciura</a> has an amazing vocabulary!</p>
 <p>There are also nods to entries of the years <a href="../2000/bellard/index.html">2000/bellard</a>,
-<a href="../1989/yang/index.html">1989/yang</a>, …</p>
+<a href="../2015/yang/index.html">2015/yang</a>, …</p>
 <p>…We’ll stop spouting spoilers now. Have fun exploring all the entries!</p>
 <h2 id="remarks-on-some-of-submissions-that-did-not-win">Remarks on some of submissions that did not win</h2>
 <p>The number of entries that would have made it into the final rounds