fix(card): rename misnamed instance of background color mod (#2417)

The custom mod property for the background color was named incorrectly
in one of the declarations. It already exists and is used by the rest
of the file with the correct name, so this is more of a bug fix than
an overall renaming.

Author: jawinn

components/card/index.css

@@ -137,11 +137,16 @@ governing permissions and limitations under the License.
 		--highcontrast-card-border-color,
 		var(--mod-card-border-color, var(--spectrum-card-border-color))
 	);
+	/* @deprecation --mod-spectrum-card-background-color has been renamed to
+		--mod-card-background-color. The fallback will be removed in a future version. */
 	background-color: var(
 		--highcontrast-card-background-color,
 		var(
-			--mod-spectrum-card-background-color,
-			var(--spectrum-card-background-color)
+			--mod-card-background-color,
+			var(
+				--mod-spectrum-card-background-color,
+				var(--spectrum-card-background-color)
+			)
 		)
 	);