Add 'zcal' social icon to the list in papermod-icons.md

[aaccioly]

What does this PR change? What problem does it solve?

Just adding zcal icon to the Social-Icons / Share-Icons page of the Demo site.
Please merge #1746 before merging this one.

Also, I haven't found a way to update the Wiki, so, after merging this one, it may need to be reflected at the Wiki's Icon page

Was the change discussed in an issue or in the Discussions before?

No

PR Checklist

• This change adds/updates translations and I have used the template present here.
• I have enabled maintainer edits for this PR.
• I have verified that the code works as described/as intended.
• This change adds a Social Icon which has a permissive license to use it.
• This change does not include any CDN resources/links.
• This change does not include any unrelated scripts such as bash and python scripts.
• This change updates the overridden internal templates from HUGO's repository.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud