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>

docs/nvdimm.txt

@@ -15,7 +15,7 @@ backend (i.e. memory-backend-file and memory-backend-ram). A simple
 way to create a vNVDIMM device at startup time is done via the
 following command line options:
 
- -machine pc,nvdimm
+ -machine pc,nvdimm=on
  -m $RAM_SIZE,slots=$N,maxmem=$MAX_SIZE
  -object memory-backend-file,id=mem1,share=on,mem-path=$PATH,size=$NVDIMM_SIZE,readonly=off
  -device nvdimm,id=nvdimm1,memdev=mem1,unarmed=off