Add conn2i back to vignette

[jwokaty]

```{r, fig.small = TRUE, fig.cap = "Example conn2i", echo = FALSE}
plot(coex2i)
```

Over time, the above chunk got separated from the original section it was a part of. I thought it was cruft and removed it, but it should be in the section for isomorphism.

[jwokaty]

Fixed with https://github.com/Bioconductor/RBGL/commit/d75731607d635ed05a9dd2e364b88de063cccd31