[document] fix a minor issue

Author: amin1377

doc/src/vpr/command_line_usage.rst

@@ -1299,6 +1299,7 @@ The following options are only valid when the router is in timing-driven mode (t
     **Default:** ``1.2``
 
 .. option:: --router_profiler_astar_fac <float>
+    
     Controls the directedness of the timing-driven router's exploration when doing router delay profiling of an architecture.
     The router delay profiling step is currently used to calculate the place delay matrix lookup.
     Values between 1 and 2 are resonable; higher values trade some quality for reduced run-time.