Updated MaterialTextView's documentation with the right package name of the component.

Resolves #541

GIT_ORIGIN_REV_ID=c9d1e674187c3fe23b7f209cbbbc8008e348592b
PiperOrigin-RevId: 266214327

Author: gabrielemariotti

docs/components/MaterialTextView.md

@@ -27,14 +27,18 @@ reference.
   xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="match_parent"
   android:layout_height="match_parent">
-    <MaterialTextView
+    <TextView
       android:id="@+id/text_view_id"
       android:layout_height="wrap_content"
       android:layout_width="wrap_content"
       android:text="@string/hello" />
 </LinearLayout>
 ```
 
+Note: If you use our full themes (which we recommend), `TextView` will
+auto-inflate to `MaterialTextView`, otherwise, you will need to specify
+`<com.google.android.material.textview` in your xml.
+
 ### Attributes
 
 `MaterialTextView` supports all of the standard attributes that can be changed