website: Fixed a typo in gem5 Documentation

The typo is in section

  Modifying/Extending - Creating SimObjects in the memory system

Change-Id: Ic41093d196a32b91281349729b832b522e11e098
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/61742
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>