Readme: Update footer fallback icon

_includes/footer_custom.html

@@ -2,11 +2,11 @@
   <br>
   <p class="text-small text-grey-dk-000 mb-0">Explore My GNOME Extensions</p>
   <div id='container1'>
-    <img src='./assets/images/footer/bhc-logo.png' width="4%" style='float: left; margin-right: 0.5rem;'/>
+    <img src='./assets/images/footer/bhc-logo.png' onerror="this.onerror=null;this.src='../assets/images/footer/bhc-logo.png'" width="4%" style='float: left; margin-right: 0.5rem;'/>
     <p class="text-small text-grey-dk-000 mb-0"><a href="https://extensions.gnome.org/extension/5724/battery-health-charging/">Battery Health Charging:</a> An extension for setting Charging threshold for batteries on laptops</p>
   </div>
   <div id='container2'>
-    <img src='./assets/images/footer/bbm-logo.png' width="4%" style='float: left; margin-right: 0.5rem;'/>
+    <img src='./assets/images/footer/bbm-logo.png' onerror="this.onerror=null;this.src='../assets/images/footer/bbm-logo.png'" width="4%" style='float: left; margin-right: 0.5rem;'/>
     <p class="text-small text-grey-dk-000 mb-0"><a href="https://extensions.gnome.org/extension/5724/bluetooth-battery-level/">Bluetooth Battery Meter:</a> An extension featuring indicator icons in system tray</p>
   </div>
   <br>