You are right there should be a dot at the r4 /L1/C1 junction as R4 should connect to the trigger line. I missed that when I modified the drawing. The idea is when the coil is not active (due to no spark) the R3/R4 junction (and therefore the trigger) will be about a volt higher than the trigger level. The spark will hopefully cause at least a volt or 2 of change in the coil which will drive the trigger below the threshold and trigger the 555 to extend the pulse.
Peter