fix: remove (includes samples) from overview link

[diegomarquezp]

b/404600681
The samples inside the GitHub repositories are not meant for user consumption and are better portrayed in https://cloud.google.com/docs/samples

see gpaste/5865295979479040 for final result tried locally against java-spanner. The relevant part is:

<table>
   <tr>
     <td><a href="https://cloud.google.com/spanner/docs/">Cloud Spanner Product Reference</a></td>
     <td><a href="https://github.com/googleapis/java-spanner/tree/main/">GitHub Repository</a></td>
     <td><a href="https://central.sonatype.com/artifact/com.google.cloud/google-cloud-spanner">Maven artifact</a></td>
   </tr>
 </table>