database/sql: fix doc references to old package name

It used to be package "db" but was long ago renamed
to be "sql".

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7322075

src/pkg/database/sql/doc.txt

@@ -21,7 +21,7 @@ Goals of the sql and sql/driver packages:
   Database Driver -> sql (to register) + sql/driver (implement interfaces)
 
 * Make type casting/conversions consistent between all drivers. To
-  achieve this, most of the conversions are done in the db package,
+  achieve this, most of the conversions are done in the sql package,
   not in each driver. The drivers then only have to deal with a
   smaller set of types.
 

src/pkg/database/sql/driver/driver.go

@@ -56,7 +56,7 @@ var ErrBadConn = errors.New("driver: bad connection")
 
 // Execer is an optional interface that may be implemented by a Conn.
 //
-// If a Conn does not implement Execer, the db package's DB.Exec will
+// If a Conn does not implement Execer, the sql package's DB.Exec will
 // first prepare a query, execute the statement, and then close the
 // statement.
 //
@@ -67,7 +67,7 @@ type Execer interface {
 
 // Queryer is an optional interface that may be implemented by a Conn.
 //
-// If a Conn does not implement Queryer, the db package's DB.Query will
+// If a Conn does not implement Queryer, the sql package's DB.Query will
 // first prepare a query, execute the statement, and then close the
 // statement.
 //