Update LICENSE year

[pllim]

Pretty sure hstcal existed before 2018, so I guess you are only keeping the current year, and not a range?

p.s. I mainly wanna see if the labeler works but you might as well merge this.

[jhunkeler]

If we want to do a range then there are two starting dates to pick from. 😁

The first commit on this repository:

commit f6638bbcb7605d72e17f2696305c42bb35a44e98
Author: hack <hack at stsci.edu>
Date:   Thu Mar 11 18:20:58 2010 +0000

Or the genesis of hst_calib.cl in stsdas:

commit ec9d555c5fd92591a3d72d21877486345f9a91fe
Author: sdas <sdas@fe389314-cf27-0410-b35b-8c050e845b92>
Date:   Thu Jul 31 18:31:24 1997 +0000

[pllim]

1997 sounds about right!