Use , as author separator in footer

Change-Id: Id988591fe1e5ba728c13c2e636144779223ce751
diff --git a/inst/rmarkdown/templates/revealjs_presentation/resources/default.html b/inst/rmarkdown/templates/revealjs_presentation/resources/default.html
index 8910fb5..ba1885c 100644
--- a/inst/rmarkdown/templates/revealjs_presentation/resources/default.html
+++ b/inst/rmarkdown/templates/revealjs_presentation/resources/default.html
@@ -715,14 +715,7 @@
 $endfor$
 $if(theme-ids)$
     <footer id="ids-footer"><span>
-      $for(author)$
-$if(author.name)$
-    $author.name$
-$else$
-    $author$
-$endif$
-  ${ sep }·
-$endfor$: $title$ · $date$</span></footer>
+      $for(author)$$if(author.name)$$author.name$$else$$author$$endif$${ sep }, $endfor$ · $title$ · $date$</span></footer>
 $endif$
   </body>
 </html>