🐛 fix header border in markdown style

[uzimaru0000]

In current

$ tv example.json --style markdown -a center 
|id|          name          |    username    |          email          |address|        phone        |   website   |company|
|--|------------------------|----------------|-------------------------|-------|---------------------|-------------|-------|
| 1|      Leanne Graham     |      Bret      |    Sincere@april.biz    |  ...  |1-770-736-8031 x56442|hildegard.org|  ...  |
| 2|      Ervin Howell      |    Antonette   |    Shanna@melissa.tv    |  ...  | 010-692-6593 x09125 |anastasia.net|  ...  |
| 3|    Clementine Bauch    |    Samantha    |    Nathan@yesenia.net   |  ...  |    1-463-123-4447   | ramiro.info |  ...  |
| 4|    Patricia Lebsack    |    Karianne    |Julianne.OConner@kory.org|  ...  |  493-170-9623 x156  |   kale.biz  |  ...  |
| 5|    Chelsey Dietrich    |     Kamren     | Lucio_Hettinger@annie.ca|  ...  |    (254)954-1289    | demarco.info|  ...  |
| 6|  Mrs. Dennis Schulist  |Leopoldo_Corkery| Karley_Dach@jasper.info |  ...  | 1-477-935-8478 x6430|   ola.org   |  ...  |
| 7|     Kurtis Weissnat    |  Elwyn.Skiles  |  Telly.Hoeger@billy.biz |  ...  |     210.067.6132    |   elvis.io  |  ...  |
| 8|Nicholas Runolfsdottir V|  Maxime_Nienow |   Sherwood@rosamond.me  |  ...  |  586.493.6943 x140  | jacynthe.com|  ...  |
| 9|     Glenna Reichert    |    Delphine    | Chaim_McDermott@dana.io |  ...  | (775)976-6794 x41206|  conrad.com |  ...  |
|10|   Clementina DuBuque   | Moriah.Stanton |  Rey.Padberg@karina.biz |  ...  |     024-648-3804    | ambrose.net |  ...  |

Markdown center is not applied