Fix typo in -P help for ndpiReader

[torres-miguel]

Please sign (check) the below before submitting the Pull Request:

• I have signed the ntop Contributor License Agreement at https://github.com/ntop/legal/blob/main/individual-contributor-licence-agreement.md
• I have read the contributing guidelines at https://github.com/ntop/nDPI/blob/dev/CONTRIBUTING.md
• I have updated the documentation (in doc/) to reflect the changes made (if applicable)

Link to the related issue:

Describe changes:
Fixes a minor typo in the help message for the -P option in ndpiReader.
Goes from d = max num reported payloads to e = max num reported payloads (it repeated a d)

[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

[IvanNardi]

Thank you