docs/nvdimm: Update nvdimm option value in machine example

docs/nvdimm: Update nvdimm option value in machine example

Update nvdimm option value in example command from "-machine pc,nvdimm"
to "-machine pc,nvdimm=on" as former complains with the below error:

"qemu-system-x86_64: -machine pc,nvdimm: Expected '=' after parameter 'nvdimm'"

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Message-Id: <20210923103015.135262-1-pankaj.gupta.linux@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>