Merge: Fix nitunit for `String::format`

Since the code block was badly indented, it was not tested by `nitunit`.

Once the indentation was corrected, `nitunit` found that the test was failing.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

Pull-Request: #1342
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/standard/string.nit
Simple merge