message: Fix typo for MBF
diff: | Prefix | Command |
  | ------ | ---------------- |
  | `MRM` | `\mathrm{${1}}` |
  -| `MBF` | `\mathbb{${1}}` |
  +| `MBF` | `\mathbf{${1}}` |
  | `MBB` | `\mathbb{${1}}` |
  | `MCA` | `\mathcal{${1}}` |
  | `MIT` | `\mathit{${1}}` |
  