Fails to properly clean _minted directory under ParU/Doc/

[svillemot]

At least under Debian, the build system for ParU documentation creates a directory called ParU/Doc/_minted.

That directory is not properly cleant by ParU/Doc/Makefile, because the latter looks for a _minted-paru* pattern.

I suggest to replace that pattern with _minted* (in two places in the Makefile), so as to cover all cases.