fix(tree): Fix tree nested data source

fix docs

Author: tinayuangao

src/material-experimental/tree/data-source/nested-data-source.ts

@@ -28,7 +28,7 @@ export class MatTreeNestedDataSource<T> implements DataSource<T> {
   set data(value: T[]) { this._data.next(value); }
 
   connect(collectionViewer: CollectionViewer): Observable<T[]> {
-    return merge([collectionViewer.viewChange, this._data])
+    return merge(...[collectionViewer.viewChange, this._data])
       .pipe(map(() => {
         return this.data;
       }));

src/material-experimental/tree/tree.md

@@ -5,7 +5,7 @@ This tree builds on the foundation of the CDK tree and uses a similar interface
 data source input and template, except that its element and attribute selectors will be prefixed
 with `mat-` instead of `cdk-`.
 
-There are two types of trees: Flat tree and Nested Tree. The DOM structures are different for these
+There are two types of trees: Flat tree and nested tree. The DOM structures are different for these
 two types of trees.
 
 #### Flat tree