small doc fix COlumns -> Columns

sqlx.go

@@ -723,7 +723,7 @@ func SliceScan(r ColScanner) ([]interface{}, error) {
 // (for instance, if you're building an interface for an SQL server that
 // executes SQL from input).  Please do not use this as a primary interface!
 // This will modify the map sent to it in place, so reuse the same map with
-// care.  COlumns which occur more than once in the result will overwrite
+// care.  Columns which occur more than once in the result will overwrite
 // eachother!
 func MapScan(r ColScanner, dest map[string]interface{}) error {
 	// ignore r.started, since we needn't use reflect for anything.