In Nbconvert Support.ipynb raw cell example should use \frac not \\frac

At least in my install, \frac{} in the last cell produces correct html, not \\frac.
Config:
```bash
%%bash
python -V
jupyter --version
jupyter nbconvert --version
```

```
Python 3.5.3
4.3.0
5.1.1
```
Oh, the browser does not seem to make a difference (Firefox, Chrome tested).

Open Support.ipynb and inspect the last raw-cell example, then render the notebook with the reported nbconvert setup to confirm the current HTML output. Update the example to use the escaping that produces the intended fraction, and verify the rendered HTML displays it correctly.

Written by the indexing model from the issue text.