database/sql: fix spelling mistake in tests

Change-Id: I04e150d4e4123aad2f277e5c6e9f2abd15628a28
Reviewed-on: https://go-review.googlesource.com/37941
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql_test.go

@@ -1161,7 +1161,7 @@ func TestParentStmtOutlivesTxStmt(t *testing.T) {
 	}
 }
 
-// Test that tx.Stmt called with a statment already
+// Test that tx.Stmt called with a statement already
 // associated with tx as argument re-prepares the same
 // statement again.
 func TestTxStmtFromTxStmtRePrepares(t *testing.T) {