riscv: cpu: fu740: Fix typo of date

Fixed the typo of date of copyright declaration.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/riscv/cpu/fu740/spl.c

@@ -1,6 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0+
 /*
- * Copyright (C) 2020-201 SiFive, Inc
+ * Copyright (C) 2020-2021 SiFive, Inc
  * Pragnesh Patel <pragnesh.patel@sifive.com>
  */