.gitignore: update documentation directory name

In the yocto-docs repository, the documentation is stored in a directory
called "documentation". In this repository the documentation is stored in
"doc".

This update allows git to properly ignore generated documentation.

Signed-off-by: Trevor Woerner <trevor.woerner@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>