Add package needed to build a pdf file

Summary: Added python3-sphinx-bootstrap-theme which is needed for `make latexpdf`.

Test Plan: Install python3-sphinx-bootstrap-theme and build a pdf from source.

Reviewers: lynorian, tsimonq2, wxl

Differential Revision: https://phab.lubuntu.me/D31

README.md

@@ -9,7 +9,7 @@ The documentation is powered by [Sphinx](http://www.sphinx-doc.org/en/stable/) a
 Run the following command to install the needed dependencies:
 
 ```
-sudo apt install python3-pip python3-sphinx latexmk texlive texlive-formats-extra arcanist python3-sphinx-rtd-theme 
+sudo apt install python3-pip python3-sphinx latexmk texlive texlive-formats-extra arcanist python3-sphinx-rtd-theme python3-sphinx-bootstrap-theme
 ```
 
 You can see the options for building the documentation by running `make help`. You can make the html version by running `make html`. You can make a pdf version of the lubuntu manual with `make latexpdf`. If you just want to check for warnings or errors building run `make dummy`.