Fix dot graph reader

The correct type for the dynamic property map key is actually
graph_property_tag, not multigraph_t*. It looks like graph property maps
only store a single value...
2 jobs for bugfix/graphviz-read-graph-properties in 114 minutes and 4 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #4868
amd64
job_clang_amd64

01:54:04

passed #4867
amd64
job_gcc_amd64

01:31:16