docs/manual: fix bash export syntax in example code

To export a variable in .bashrc we shouldn't put a '$' at the
beginning of the line, and there must be a '=' between key and value.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>