[RISC-V] Fix broken link to ELF psABI documentation (PR#10851)

Fixes a broken link to the RISC-V ELF psABI documentation, which has
been moved to the non-ISA org.

Signed-off-by: hasheddan <[email protected]>

Author: dra27

asmcomp/riscv/proc.ml

@@ -286,7 +286,8 @@ let prologue_required fd =
   frame_required fd
 
 (* See
-   https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md *)
+   https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc
+*)
 
 let int_dwarf_reg_numbers =
   [| 10; 11; 12; 13; 14; 15; 16; 17;