.pytool: Fix python command error in self introduction doc

.pytool: Fix python command error in self introduction doc

Fix misspelling of word "Prerequisites".

In the step 5 of the Prerequisites section, the python command was missing
the "-r" option, what the command really meant was to install from the given
requirements file, so fix that.

Cc: sean.brogan@microsoft.com
Cc: Bret.Barkelew@microsoft.com
Cc: michael.d.kinney@intel.com
Cc: gaoliming@byosoft.com.cn
Signed-off-by: Chao Li <lichao@loongson.cn>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>