Fixed sites JSON import

[manuelceroni]

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 guide lines https://github.com/ntop/ntopng/blob/dev/CONTRIBUTING.md
• I have updated the documentation (in doc/src/) to reflect the changes made (if applicable)

Link to the related issue:

Describe changes:
Fixed sites JSON import