database/sql: fix docs to correctly refer to time.RFC3339Nano

It mentions time.Format3339Nano, which isn’t defined. The
underlying code uses time.RFC3339Nano.

Updates #24542

Change-Id: Ia34ae8b66427139d9005f902c2eb60aac4bfa8c6
Reviewed-on: https://go-review.googlesource.com/102607
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -2856,7 +2856,7 @@ func rowsColumnInfoSetupConnLocked(rowsi driver.Rows) []*ColumnType {
 //
 // Source values of type time.Time may be scanned into values of type
 // *time.Time, *interface{}, *string, or *[]byte. When converting to
-// the latter two, time.Format3339Nano is used.
+// the latter two, time.RFC3339Nano is used.
 //
 // Source values of type bool may be scanned into types *bool,
 // *interface{}, *string, *[]byte, or *RawBytes.