Fix doc typo, add defaultNamespace method.

Author: rdblue

sql/catalyst/src/main/java/org/apache/spark/sql/catalog/v2/NamespaceChange.java

@@ -23,7 +23,7 @@
  * <pre>
  *   import NamespaceChange._
  *   val catalog = Catalogs.load(name)
- *   catalog.namespaces.alterNamespace(ident,
+ *   catalog.alterNamespace(ident,
  *       setProperty("prop", "value"),
  *       removeProperty("other_prop")
  *     )

sql/catalyst/src/main/java/org/apache/spark/sql/catalog/v2/SupportsNamespaces.java

@@ -37,6 +37,20 @@
  */
 public interface SupportsNamespaces extends CatalogPlugin {
 
+  /**
+   * Return a default namespace for the catalog.
+   * <p>
+   * When this catalog is set as the current catalog, the namespace returned by this method will be
+   * set as the current namespace.
+   * <p>
+   * The namespace returned by this method is not required to exist.
+   *
+   * @return a multi-part namespace
+   */
+  default String[] defaultNamespace() {
+    return new String[0];
+  }
+
   /**
    * List top-level namespaces from the catalog.
    * <p>