COPYING.MPL2: Update text from mozilla.org?

Dear Eigen team,

I noticed that the Mozilla guys have updated the MPL2 license text a bit (line 360, http -> https); I thought you might want to update COPYING.MPL2 as well.

References:

• https://www.mozilla.org/en-US/MPL/
• https://www.mozilla.org/media/MPL/2.0/index.48a3fe23ed13.txt

Best regards, Thomas