doc: add missing paren (pointed out by nbjoerg)

R=rsc, r
CC=golang-dev
https://golang.org/cl/1983045

doc/effective_go.html

@@ -2603,7 +2603,7 @@ func CubeRoot(x float64) float64 {
         }
     }
     // A million iterations has not converged; something is wrong.
-    panic(fmt.Sprintf("CubeRoot(%g) did not converge", x)
+    panic(fmt.Sprintf("CubeRoot(%g) did not converge", x))
 }
 </pre>