Fix a typo in the GPU Docs

Using the "-pk" switch explicitly allows for setting of the number of

GPUs/node to use and additional options.  Its syntax is the same as

same as the "package gpu" command.  See the :doc:`package <package>`

the "package gpu" command.  See the :doc:`package <package>`

command doc page for details, including the default values used for

all its options if it is not specified.