C-SKY: Fix copyright of csky-modes.def.

C-SKY: Fix copyright of csky-modes.def.

The incorrect copyright comment format causes build error:
builddir/source//gcc/gcc/config/csky/csky-modes.def: In function ‘void create_modes()’:
builddir/source//gcc/gcc/config/csky/csky-modes.def:1:4: error: ‘C’ was not declared in this scope
 ;; C-SKY extra machine modes.
    ^
builddir/source//gcc/gcc/config/csky/csky-modes.def:1:6: error: ‘SKY’ was not declared in this scope
 ;; C-SKY extra machine modes.
      ^
builddir/source//gcc/gcc/config/csky/csky-modes.def:2:16: error: ‘Copyright’ was not declared in this scope
 ;; Copyright (C) 2018-2021 Free Software Foundation, Inc.
                ^
builddir/source//gcc/gcc/config/csky/csky-modes.def:3:4: error: ‘Contributed’ was not declared in this scope
 ;; Contributed by C-SKY Microsystems and Mentor Graphics.
    ^

gcc/ChangeLog:
	* config/csky/csky-modes.def : Fix copyright.