Inconsistent naming convention in the Gray-Scott example

[manikantateja973]

In the README.md file under the builtin gray_scott package (PATH: builtin/builtin/gray_scott/README.md), the scspkg that was created in the first line (which is gray-scott) does not match with name that's used later (gray_scott) under the Installation section.

Pointed out the issue as commented lines below

scspkg create gray-scott                    #gray-scott is different from gray_scott used below
cd `scspkg pkg src gray-scott`           #gray-scott is different from gray_scott used below
git clone https://github.com/pnorbert/adiosvm
cd adiosvm/Tutorial/gs-mpiio
mkdir build
pushd build
cmake ../ -DCMAKE_BUILD_TYPE=Release
make -j8
export GRAY_SCOTT_PATH=`pwd`
scspkg env set gray_scott GRAY_SCOTT_PATH="${GRAY_SCOTT_PATH}"
scspkg env prepend gray_scott PATH "${GRAY_SCOTT_PATH}"               #gray_scott is different from gray-scott used above
module load gray_scott
spack load mpi adios2