Fix group tag for SVG plotting

This was previously "svg:g", but the SVG is declared with only
a "default" namespace, not an "svg:" namespace (looks like
xmlns:svg="http://www.w3.org/2000/svg" in the svg tag).

This means you cannot use svg: as a NS in the document. It's not
needed anywaY: the default is already correct.

Moreover, mismatching <svg:g> and </g> is always invalid.

common/plotters/SVG_plotter.cpp

@@ -292,7 +292,7 @@ void SVG_PLOTTER::StartBlock( void* aData )
 {
     std::string* idstr = reinterpret_cast<std::string*>( aData );
 
-    fputs( "<svg:g ", outputFile );
+    fputs( "<g ", outputFile );
     if( idstr )
         fprintf( outputFile, "id=\"%s\"", idstr->c_str() );