Problem with adding a new google font in Enfold child theme's functions.php

Hello guys,
I’m using the Enfold child theme, and I am trying to add a new google font. So as suggested by you to others, I inserted the following code to functions.php from my WP Editor section:

add_filter( ‘avf_google_heading_font’, ‘avia_add_heading_font’);
function avia_add_heading_font($fonts)
{
$fonts[‘Source Sans Pro’] = ‘Source Sans Pro:400,600,800′;
$fonts[‘Raleway’] = ‘Raleway:400,600,800′;
return $fonts;
}

But after saving it my browser says there’s a
“Parse error: syntax error, unexpected ‘avf_google_content_font’ (T_STRING)” in my /enfold-child/functions.php on line 15

What can I do to fix this?
Thanks a lot!

Davide