Correctly style image loading indicator for various themes

Wrong class name was being used

extensions/image-preview/media/main.css

@@ -106,10 +106,10 @@ body img {
 	margin: 1em;
 }
 
-.vscode-dark .loading {
+.vscode-dark .loading-indicator {
 	background-image: url('./loading-dark.svg');
 }
 
-.vscode-high-contrast .loading {
+.vscode-high-contrast .loading-indicator {
 	background-image: url('./loading-hc.svg');
 }