Update CONTRIBUTING.md

Fix typo and update number of questions

Signed-off-by: Leandro Siqueira <leandro.siqueira@gmail.com>

CONTRIBUTING.md

@@ -166,7 +166,7 @@ However, there might be a way to implement that feature *on top of* Docker.
   <tr>
     <td>Stack Overflow</td>
     <td>
-      Stack Overflow has over 7000K Docker questions listed. We regularly
+      Stack Overflow has over 17000 Docker questions listed. We regularly
       monitor <a href="https://stackoverflow.com/search?tab=newest&q=docker" target="_blank">Docker questions</a>
       and so do many other knowledgeable Docker users.
     </td>