In KCM, you can set 1-character long prefixes for web shortcuts. However these won't work in Krunner search. Furthermore, 1 character long prefixes can be used in both chrome and firefox, so makes sense to allow them to be used in Krunner as well, for consistency's sake (and maybe at some point KDE will gain the capability to sync its search engines with firefox and chrome)
You can do this, the changes are just not immediately applied in KRunner :-) *** This bug has been marked as a duplicate of bug 391243 ***