Update progress demos

src/progress/snippets/progress-buffering-demo.html

@@ -1,5 +1,5 @@
 <style>
-  .progress-demo {
+  .demo-progress__progress-buffering .mdl-progress {
     width: 50vw;
     max-width: 260px;
   }

src/progress/snippets/progress-default-demo.html

@@ -1,8 +1,7 @@
 <style>
-  .progress-demo {
+  .demo-progress__progress-default .mdl-progress {
     width: 50vw;
     max-width: 260px;
   }
 </style>
-
 {% include "progress-default.html" %}

src/progress/snippets/progress-indeterminate-demo.html

@@ -1,9 +1,7 @@
 <style>
-  .progress-demo {
+  .demo-progress__progress-indeterminate .mdl-progress {
     width: 50vw;
     max-width: 260px;
-    margin: 60px 0;
   }
 </style>
-
 {% include "progress-indeterminate.html" %}