Fix submit button on the settings page

[samuelmanzanera]

Description

After the update of the Phoenix deps, a PR introduced a regression about the html button in the from of the settings page.

This PR fixes the HTML code.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Start a node
• Go the the settings page: http://localhost:4000/settings
• Change the reward address
• Click on the button "Save"
• The update should be have been taken

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules