docs: fix bad link for isocalendar demo [skip ci]

Author: lowlighter

source/plugins/isocalendar/README.md

@@ -8,7 +8,7 @@ The *isocalendar* plugin displays an isometric view of your commits calendar, al
       <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.isocalendar.fullyear.svg">
     </details>
     <details><summary>Half year isometric calendar</summary>
-      <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.isocalendar.fullyear.svg">
+      <img src="https://github.com/lowlighter/lowlighter/blob/master/metrics.plugin.isocalendar.svg">
     </details>
     <img width="900" height="1" alt="">
   </td>