message: Fix minor errors in snippets-doc.md
diff: | `@<` | `\leq` |
  | `@>` | `\geq` |
  | `@2` | `\sqrt{$1}$0` |
  -| `$@I` | `\int_{$1}^{$2}$0` |
  -| `@| ` | `\Big | ` |
  +| `@I` | `\int_{$1}^{$2}$0` |
  +| <code>@&#124;</code> | <code>\Big &#124;</code> |
  | `@\` | `\setminus` |
  | `@+` | `\bigcup` |
  | `@-` | `\bigcap` |
  