docs: update calendar for 20.1.0-rc3

Part-of: <mesa/mesa!5026>

docs/release-calendar.html

@@ -72,13 +72,7 @@ if you'd like to nominate a patch in the next stable release.
 <td>Last planned release of the 20.0 series</td>
 </tr>
 <tr>
-<td rowspan="2">20.1</td>
-<td>2020-05-13</td>
-<td>20.1.0-rc3</td>
-<td>Eric Engestrom</td>
-<td></td>
-</tr>
-<tr>
+<td rowspan="1">20.1</td>
 <td>2020-05-20</td>
 <td>20.1.0-rc4</td>
 <td>Eric Engestrom</td>