Remove trailing whitespaces in new references

The "Booked N Histogram(s) : ..." message always contains a trailing space at the end of the line, which ends up being propagated to new references. This whitespace has no significance, so with this change it is stripped, which should reduce diffs (due to editors stripping trailing whitespace).

No immediate reference updates are needed, but the next time they are done the trailing whitespaces will be removed.