index.html: Fix link text

Author: mkoeppe

index.html

@@ -15,7 +15,7 @@ <h1 style="font-size: 5.9vw";>
           <text x="637" y="170" text-anchor="middle">GitHub repository</text>
         </a>
         <a xlink:href="docs/10.6" xlink:title="documentation">
-          <text x="425" y="1290" text-anchor="middle">documentation 12.6</text>
+          <text x="425" y="1290" text-anchor="middle">documentation 10.6.x</text>
       </svg>
     </div>
     <div style="position: absolute; top: 8px; left: 16px; ">passagemath</div>