database/sql: fix wrong method name in description

Change-Id: Ie6a88b70d7c45c59995ee2f57fb28f9a3cbb404d
Reviewed-on: https://go-review.googlesource.com/49470Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -1813,7 +1813,7 @@ func (tx *Tx) Rollback() error {
 	return tx.rollback(false)
 }
 
-// Prepare creates a prepared statement for use within a transaction.
+// PrepareContext creates a prepared statement for use within a transaction.
 //
 // The returned statement operates within the transaction and will be closed
 // when the transaction has been committed or rolled back.