L1 convergence in the Birkhoff ergodic theorem on a finite measure space ($A_{n}f\to f^{\ast }$ in $L^1$)

On a finite measure space, the convergence in the Birkhoff ergodic theorem also holds in $L^1$. Prove it first for bounded truncations by the dominated convergence theorem, then use the $L^1$ contraction $\Vert A_{n}h{\Vert }_1\le \Vert h{\Vert }_1$ and Fatou lemma to remove the truncation.