Fix wrong path for "AboutThisUser" callback in ShowGroupMembers

Since caller() returns where the sub was called in ShowPrincipal,
9d852519 unintentionally moved this callback to
/Elements/ShowPrincipal/AboutThisUser from
/Ticket/Elements/ShowGroupMembers/AboutThisUser -- move it back.

Fies I#30098
Merge branch '4.2/fix-callback-path' into 4.2-trunk