Update civlog documentation URL to point to main branch
Once freeciv-3.3 development goes to 'main' branch ( #44375 ), report.c URL pointing to 'master' branch needs to be updated.
Also data/themes/gui-sdl2/human/theme.themespec mention 'master' branch.
Once freeciv-3.3 development goes to 'main' branch ( #44375 ), report.c URL pointing to 'master' branch needs to be updated.