python: Update gem5 url output by the simulator

This patch:
- Replaces 'http' with 'https'.
- Adds 'www'.

Change-Id: I6500a39171eb79c98754f517ff1fdd0cf60d83d0
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56649
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/main.py

@@ -310,7 +310,7 @@ def main():
 
     verbose = options.verbose - options.quiet
     if verbose >= 0:
-        print("gem5 Simulator System.  http://gem5.org")
+        print("gem5 Simulator System.  https://www.gem5.org")
         print(brief_copyright)
         print()