Typo for AgeIPO in SignalDoc

[chenandrewy]

The SampleStartYear here does not seem right:

For IPOs it looks right. Table II lists dates 1975-84. But Table IX does not list dates but it also doesn't say it's using a super tiny subset as described in Signaldoc.csv. Here's the full table:

[tomz23]

Updated in 5790257.

The table note says that it uses the same number of IPOs as the other tables, so sample period should be the same, too. In addition, same note as for IndIPO applies: Sample end year should be extended by three years since the strategy follows stocks for three years.