docs: Syntax error in Nested tree section in src/lib/tree/tree.md

[AndreasFagschlunger]

Bug, feature request, or proposal:

Bug/feature:

Quite obvious mat-nested-tree-node need to be closed by </mat-nested-tree-node>?

<mat-tree>
   <mat-nested-tree-node>
     parent node
     <mat-nested-tree-node> -- child node1 </mat-tree-node>
     <mat-nested-tree-node> -- child node2 </mat-tree-node>
   </mat-nested-tree-node>
</mat-tree>

What is the expected behavior?

The mat-nested-tree-node should be closed with </mat-nested-tree-node>.

What is the current behavior?

Tag opens with <mat-nested-tree-node> and closes with </mat-tree-node>.

What are the steps to reproduce?

VIsit https://material.angular.io/components/tree/overview and scroll to Nested tree section.

What is the use-case or motivation for changing an existing behavior?

The documentation contains syntactical incorrect snippets.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material 6.0.0, all Browsers

Is there anything else we should know?

I actually read the docs.

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