Fix graphviz file header (#261)

The two header comments were missing trailing newlines.

This behavior is introduced by v0.18 of the graphviz Python package;
where as v0.17 did include the newline automatically.

Closes #258

src/wireviz/Harness.py

@@ -151,8 +151,8 @@ class Harness:
 
     def create_graph(self) -> Graph:
         dot = Graph()
-        dot.body.append(f"// Graph generated by {APP_NAME} {__version__}")
-        dot.body.append(f"// {APP_URL}")
+        dot.body.append(f"// Graph generated by {APP_NAME} {__version__}\n")
+        dot.body.append(f"// {APP_URL}\n")
         dot.attr(
             "graph",
             rankdir="LR",