docs(cdk/dialog): improve dialog example contrast (#33334)

(cherry picked from commit ca71bbe)

Author: BHUVANSH855

src/components-examples/cdk/dialog/cdk-dialog-data/cdk-dialog-data-example-dialog.css

@@ -1,6 +1,7 @@
 :host {
   display: block;
   background: #fff;
+  color: CanvasText;
   border-radius: 8px;
   padding: 8px 16px;
 }

src/components-examples/cdk/dialog/cdk-dialog-overview/cdk-dialog-overview-example-dialog.css

@@ -1,6 +1,7 @@
 :host {
   display: block;
   background: #fff;
+  color: CanvasText;
   border-radius: 8px;
   padding: 8px 16px 16px;
 }

src/components-examples/cdk/dialog/cdk-dialog-styling/cdk-dialog-styling-example-dialog.css

@@ -1,6 +1,7 @@
 :host {
   display: block;
   background: #fff;
+  color: CanvasText;
   border-radius: 8px;
   padding: 16px;
   max-width: 500px;