RISC-V: Update the doc to match ISA manual

ISA manual use funct* rather than func*[1] (e.g. funct7 rather than func7),
and I realized that may something I typo at beginning when I write the patch
for `.insn` support...:P

[1] https://github.com/riscv/riscv-isa-manual/blob/main/src/rv32.adoc#integer-register-register-operations
1 file changed