minor improvement in docs

Author: hosseinmoein

docs/HTML/DataFrame.html

@@ -298,15 +298,7 @@ <H2><font color="blue">API Reference with code samples</font></H2>
     </tr>
 
     <tr class="item" onmouseover="this.style.backgroundColor='#ffff66';" onmouseout="this.style.backgroundColor='#d4e3e5';">
-      <td title="Performs group-by by one column"><a href="https://htmlpreview.github.io/?https://github.com/hosseinmoein/DataFrame/blob/master/docs/HTML/groupby.html">groupby1( )<BR>groupby1_async( )</a></td>
-    </tr>
-
-    <tr class="item" onmouseover="this.style.backgroundColor='#ffff66';" onmouseout="this.style.backgroundColor='#d4e3e5';">
-      <td title="Performs group-by by two column"><a href="https://htmlpreview.github.io/?https://github.com/hosseinmoein/DataFrame/blob/master/docs/HTML/groupby.html">groupby2( )<BR>groupby2_async( )</a></td>
-    </tr>
-
-    <tr class="item" onmouseover="this.style.backgroundColor='#ffff66';" onmouseout="this.style.backgroundColor='#d4e3e5';">
-      <td title="Performs group-by by two column"><a href="https://htmlpreview.github.io/?https://github.com/hosseinmoein/DataFrame/blob/master/docs/HTML/groupby.html">groupby3( )<BR>groupby3_async( )</a></td>
+      <td title="Performs group-by by one column"><a href="https://htmlpreview.github.io/?https://github.com/hosseinmoein/DataFrame/blob/master/docs/HTML/groupby.html">groupby1( )<BR>groupby1_async( )<BR>groupby2( )<BR>groupby2_async( )<BR>groupby3( )<BR>groupby3_async( )</a></td>
     </tr>
 
     <tr class="item" onmouseover="this.style.backgroundColor='#ffff66';" onmouseout="this.style.backgroundColor='#d4e3e5';">