fix: Apply margin-top to .panel instead of margin-bottom to .panel-tabs

[gadenbuie]

This will now work as expected

::: panelset
::: {.panel name="Install" .m-0 .p-3 .border .border-1}
Install xaringanExtra from [CRAN][cr] in the usual way.

```{.r}
install.packages("xaringanExtra")
```

Or install the development version from [GitHub][gh] or [r-universe][ru].

```{.r}
pak::pak("gadenbuie/xaringanExtra")
```

```{.r}
install.packages("xaringanExtra", repos = c("https://gadenbuie.r-universe.dev", "https://cran.r-project.org"))
```
:::
:::