docs(material/tree): fixed mixed node types

Fixes docs that were combining flat and nested tree nodes.

(cherry picked from commit 1d5a6a2)

Author: crisbeto

src/components-examples/material/tree/tree-nested-child-accessor-overview/tree-nested-child-accessor-overview-example.css

@@ -21,6 +21,6 @@
  * Leaf nodes need to have padding so as to align with other non-leaf nodes
  * under the same parent.
  */
-.example-tree div[role=group] > .mat-tree-node {
+.example-tree div[role=group] > .mat-nested-tree-node {
   padding-left: 40px;
 }

src/components-examples/material/tree/tree-nested-child-accessor-overview/tree-nested-child-accessor-overview-example.html

@@ -2,9 +2,9 @@
   <!-- This is the tree node template for leaf nodes -->
   <!-- There is inline padding applied to this node using styles.
     This padding value depends on the mat-icon-button width. -->
-  <mat-tree-node *matTreeNodeDef="let node">
+  <mat-nested-tree-node *matTreeNodeDef="let node">
     {{node.name}}
-  </mat-tree-node>
+  </mat-nested-tree-node>
   <!-- This is the tree node template for expandable nodes -->
   <mat-nested-tree-node
       *matTreeNodeDef="let node; when: hasChild"

src/components-examples/material/tree/tree-nested-overview/tree-nested-overview-example.css

@@ -21,6 +21,6 @@
  * Leaf nodes need to have padding so as to align with other non-leaf nodes
  * under the same parent.
  */
-.example-tree div[role=group] > .mat-tree-node {
+.example-tree div[role=group] > .mat-nested-tree-node {
   padding-left: 40px;
 }

src/components-examples/material/tree/tree-nested-overview/tree-nested-overview-example.html

@@ -2,9 +2,9 @@
   <!-- This is the tree node template for leaf nodes -->
   <!-- There is inline padding applied to this node using styles.
     This padding value depends on the mat-icon-button width. -->
-  <mat-tree-node *matTreeNodeDef="let node">
+  <mat-nested-tree-node *matTreeNodeDef="let node">
     {{node.name}}
-  </mat-tree-node>
+  </mat-nested-tree-node>
   <!-- This is the tree node template for expandable nodes -->
   <mat-nested-tree-node
       *matTreeNodeDef="let node; when: hasChild"