Update card-demo.html (#8042)

#7241 was a little heavy handed in removing all md prefixes.

Author: barnesma

src/demo-app/card/card-demo.html

@@ -7,7 +7,7 @@
     <mat-card-title-group>
         <mat-card-title>Card with title</mat-card-title>
         <mat-card-subtitle>Subtitle</mat-card-subtitle>
-        <img mat-card-mat-image>
+        <img mat-card-md-image>
     </mat-card-title-group>
   </mat-card>