American Water Works, parent of St. Louis County Water Co., made a less-than-stunning debut yesterday on the New York Stock Exchange. Its shares fell 4 percent, closing at $20.60. And that’s after parent company RWE, a German utility, cut the offering price to $21.50 from its original range of $24 to $26. RWE also sold fewer shares than it originally expected, resulting in a charge to earnings. The German company still hopes to sell its remaining 64 percent stake in American Water by year’s end.