The American economy grew at a faster pace this summer than estimated earlier. Gross domestic product grew at a 2.1 percent annual rate for the third quarter, the Commerce Department said yesterday in its second estimate of economic growth. That is a substantial boost from the 1.9 percent in the earlier estimate. While businesses pulled back on investment in the summer, likely because of fears of sluggish global growth and uncertainty over the direction of trade and monetary policies, consumers remained confident and spending high, thanks to rising wages and extremely low unemployment.