exp/sql/driver: Correct package name in comment.

IIRC, package sql used to be called db. There was one occurrence
of the old name in a comment.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5431075

src/pkg/exp/sql/driver/driver.go

@@ -7,7 +7,7 @@
 //
 // Code simply using databases should use package sql.
 //
-// Drivers only need to be aware of a subset of Go's types.  The db package
+// Drivers only need to be aware of a subset of Go's types.  The sql package
 // will convert all types into one of the following:
 //
 //   int64