Make screenshots bigger in README.md (#1799)

# What this PR does

## Which issue(s) this PR fixes

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

README.md

@@ -16,8 +16,8 @@ Developer-friendly incident response with brilliant Slack integration.
 <table>
   <tbody>
     <tr>
-      <td width="60%"><img width="100%" src="screenshot.png"></td>
-      <td><div align="center"><a href="https://grafana.com/docs/oncall/latest/mobile-app/">Android & iOS</a>:<br><img height="300px" src="screenshot_mobile.png"></div></td>
+    <td width="75%"><img src="screenshot.png"></td>
+      <td><div align="center"><a href="https://grafana.com/docs/oncall/latest/mobile-app/">Android & iOS</a>:<br><img src="screenshot_mobile.png"></div></td>
     </tr>
   </tbody>
 </table>