Fix plantuml Rst format option
Fix plantuml Rst format option

The plantuml Rst pluging was ignoring the `format` option in the `uml`
blocks, hardcoded to `png` all the time.  This did not allow for
generating `svg` images, as described in the Readme.rst.

Corrected the issue.

UHBiocomputation/pelican-plugins,rlaboiss/pelican-plugins,MarkusH/pelican-plugins,talha131/pelican-plugins,xsteadfastx/pelican-plugins,mikitex70/pelican-plugins,talha131/pelican-plugins,mikitex70/pelican-plugins,MarkusH/pelican-plugins,mortada/pelican-plugins,rlaboiss/pelican-plugins,rlaboiss/pelican-plugins,talha131/pelican-plugins,xsteadfastx/pelican-plugins,rlaboiss/pelican-plugins,UHBiocomputation/pelican-plugins,MarkusH/pelican-plugins,mikitex70/pelican-plugins,mikitex70/pelican-plugins,xsteadfastx/pelican-plugins,talha131/pelican-plugins,UHBiocomputation/pelican-plugins,MarkusH/pelican-plugins,MarkusH/pelican-plugins,mortada/pelican-plugins,mortada/pelican-plugins,talha131/pelican-plugins,xsteadfastx/pelican-plugins,mortada/pelican-plugins,mortada/pelican-plugins,UHBiocomputation/pelican-plugins