This pull request fixes a problem where the first line of content in a tab or group-tab directive is stripped in MyST.

The following in MyST will render with the first line in the tab and group-tab blocks stripped.

````{tabs}
```{tab} test
The first line.
The second line.
```
````

````{tabs}
```{group-tab} test
The first line.
The second line.
```
````

````{tabs}
```{code-tab} py
# The first line.
# The second line.
```
````

The cause of this is the index specification when getting the directive content within the TabDirective.run method.

The directive syntax in reStructuredText requires a blank line before the content, so self.content[1] is guaranteed to be a blank line. However, MyST has no such restriction; the actual content starts from self.content[1] when written as in the example above. Therefore, if one slices it as self.content[2:], the first line of the content will be stripped.

So, I modified it so that the first line of content is parsed in MyST by slicing the content from [1:]. I think that this change does not affect the parsing of content in reStructuredText.