Escape filename for pdflatex in nbconvert

This is a simple change, but it's needed otherwise filenames with spaces cause issues:

```
$ ipython3 nbconvert 12.\ Files\ and\ strings\ \(advanced\).ipynb --to latex --post PDF
[NbConvertApp] Using existing profile dir: '/Users/tom/.ipython/profile_default'
[NbConvertApp] Converting notebook 12. Files and strings (advanced).ipynb to latex
[NbConvertApp] Support files will be in 12. Files and strings (advanced)_files/
[NbConvertApp] Loaded template latex_article.tplx
[NbConvertApp] Writing 30139 bytes to ./12. Files and strings (advanced).tex
[NbConvertApp] Building PDF: `pdflatex ./12. Files and strings (advanced).tex`
/bin/sh: -c: line 0: syntax error near unexpected token `('
/bin/sh: -c: line 0: `pdflatex ./12. Files and strings (advanced).tex'
/bin/sh: -c: line 0: syntax error near unexpected token `('
/bin/sh: -c: line 0: `pdflatex ./12. Files and strings (advanced).tex'
/bin/sh: -c: line 0: syntax error near unexpected token `('
/bin/sh: -c: line 0: `pdflatex ./12. Files and strings (advanced).tex'
```