Update copy command in MkDocs build instruction to prevent copying to wrong location

[bact]

Adjust copy command in the instruction to make it still copy individual subdirs inside parser_output/mkdocs dir (but not the mkdocs dir itself), even if spdx-spec/docs/model/and spdx-spec/docs/rdf dirs are already exists.

What is wrong?

If spdx-spec/docs/model/ exists, the current command:

cp -R parser_output/mkdocs spdx-spec/docs/model 

will results this content inside the spdx-spec/docs/model/ dir:

mkdocs

which is not what we want. We want the subdirs to be copied at this level, as MkDocs expected.

What this PR do?

This PR update the command to:

mkdir -p spdx-spec/docs/model
cp -R parser_output/mkdocs/* spdx-spec/docs/model 

which will give this expected result in the spdx-spec/docs/model/ dir, no matter if the dir exists before or not:

AI			Dataset			Licensing		SimpleLicensing
Build			ExpandedLicensing	Lite			Software
Core			Extension		Security

This is new command also to prevent the case that the user may already have model subdirs inside spdx-spec/docs/model/ dir from a previous run of spec generation and want to try another run. When they make copy with the current copy command, it will results to the below content (note the mkdocs subdir at the last row at the very end):

AI			Dataset			Licensing		SimpleLicensing
Build			ExpandedLicensing	Lite			Software
Core			Extension		Security		mkdocs

This will lead to MkDocs errors.

[goneall]

LGTM