Oops, I messed up the copyright skeleton

[Michael Heerdegen]

Hello,

I got a hint that my commit replacing insignificant backquotes with
quotes messed up the "copyright" skeleton in copyright.el (quotes are
handled specially in skeletons, I just didn't know).  The corrupted
skeleton version led to inserted copyrights missing the year.

I committed a fix restoring the old version.  I hope that the corrupt
skeleton did not lead to missing copyright years in files for anybody.
The skeleton had been broken only in master since Nov 25.

Sorry for the inconvenience,

Michael.