Update code-heading placeholder language to make more consistent

src/pages/blog/[id].astro

···
 <template id="code-heading">
   <!-- slots need to be set using set:html 
    else it is treated as astro slot -->
-  <span class="lang"><Fragment set:html={"<slot>Plain Text</slot>"} /></span>
+  <span class="lang"><Fragment set:html={"<slot>.txt</slot>"} /></span>
   <span id="copied" style="visibility:hidden" role="alert">Copied!</span>
   <button id="copy"><Copy /></button>