website: Fixed compiling errors.

Accounding to the latest code, 'namespace gem5' is needed when we
define a new SimObject, otherwise compiling errors will occur like below,

error: expected class-name before '{' token ...

Change-Id: Icb8c4097420bc53d8ee88bfef260b2e4dbe60d5a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/59212
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed