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>

docs/manual/download-location.txt

@@ -17,7 +17,7 @@ set, then the value of +BR2_DL_DIR+ in the Buildroot configuration is
 overridden. The following line should be added to +<~/.bashrc>+.
 
 -----------------
- $ export BR2_DL_DIR <shared download location>
+ export BR2_DL_DIR=<shared download location>
 -----------------
 
 The download location can also be set in the +.config+ file, with the