Dax Shepard celebrated wife Kristen Bell's birthday on Sunday with a super sweet tribute.The actor and podcast host took to Instagram on the star's 41st birthday to share a gorgeous snapshot of her in overalls, boots and a quasi-cowboy hat with a bandana tied around her neck.«Happy Birthday Cowgirl,» Shepard, 46, captioned the pic. «Your beauty is only surpassed by your heart and buns.»Shepard added, «Here’s wishing you another 100 years in the saddle.