docs/zh_CN/LoongArch: Fix wrong description of FPRs Note

The Chinese translation of FPRs Note is not consistent with the original
English version, $v0/$v1 should be $fv0/$fv1, $a0/$a1 should be $fa0/$fa1,
fix them.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>