[Web] Fix quotes style in HTML shell head include

[patwork]

Info

As we use double quotes in the HTML tags, I would like to propose a fix to the platform/web/export/export_plugin.cpp code that adds headings related to icons and the PWA manifest.

Difference

Before:

<link id='-gd-engine-icon' rel='icon' type='image/png' href='mygame.icon.png' />
<link rel='apple-touch-icon' href='mygame.apple-touch-icon.png'/>
<link rel='manifest' href='mygame.manifest.json'>

After:

<link id="-gd-engine-icon" rel="icon" type="image/png" href="mygame.icon.png" />
<link rel="apple-touch-icon" href="mygame.apple-touch-icon.png"/>
<link rel="manifest" href="mygame.manifest.json">

[akien-mga]

Thanks!