fix(site): use /revdiff master in plugin example

Author: umputun

site/index.html

@@ -247,8 +247,8 @@ <h3>Install</h3>
                 <div class="flow-num">2</div>
                 <div class="flow-text">
                     <h3>Review</h3>
-                    <div class="code-block"><code>/revdiff HEAD~1</code></div>
-                    <p>Opens revdiff in an overlay. Smart ref detection when no arg given.</p>
+                    <div class="code-block"><code>/revdiff master</code></div>
+                    <p>Opens revdiff in an overlay. Reviews current branch against master.</p>
                 </div>
             </div>
             <div class="flow-step">