I'm fairly certain that Toby is correct.
in template-parts/footer, open site-info.php
Here's the code:
PHP Code:
<div class="site-info">
<a href="<?php echo esc_url( __( 'https://wordpress.org/', 'twentyseventeen' ) ); ?>"><?php printf( __( 'Proudly powered by %s', 'twentyseventeen' ), 'WordPress' ); ?></a>
</div><!-- .site-info -->