Fix Config::get call and fix Drupal best practice for SettingsForm.php

Problem/Motivation

In src/Form/SettingsForm.php the call of Config::get use two parameters, but it accept only one the name of config here is the doc page Config::get

and also there are some Drupal best practice issues:

$ drupalcsp modules/contrib/leaflet_more_maps/

FILE: modules/contrib/leaflet_more_maps/leaflet_more_maps.module
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FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
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 11 | WARNING | Global constants should not be used, move it to a class or interface
 91 | WARNING | There must be no blank line following an inline comment
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FILE: modules/contrib/leaflet_more_maps/src/Form/SettingsForm.php
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FOUND 0 ERRORS AND 2 WARNINGS AFFECTING 2 LINES
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 35 | WARNING | \Drupal calls should be avoided in classes, use dependency injection instead
 44 | WARNING | t() calls should be avoided in classes, use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
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