fix C.MODESW encoding so it doesn't overlap C.SUBW

[tariqkurd-repo]

I've changed this to the intended encoding, which is already in riscv-opcodes.
The previous one was a typo and conflicted with c.subw

[arichardson]

I was not aware of the riscv-opcodes change, maybe include pointer to riscv/riscv-opcodes#214 in the commit message when you squash and merge?

[tariqkurd-repo]

riscv/riscv-opcodes#214 is the reference. I had incorrectly typed in the encoding into this doc, which this PR fixed.