fix C.MODESW encoding so it doesn't overlap C.SUBW (#137)

I've changed this to the intended encoding, which is already in
riscv-opcodes (riscv/riscv-opcodes#214).
The previous one was a typo and conflicted with c.subw.

src/insns/wavedrom/modesw_16bit.adoc

@@ -5,7 +5,7 @@
 {reg:[
     { bits: 2, name: 0x1, attr: ['2', 'C1=1'] },
     { bits: 3, name: 0x7, attr: ['3', 'C.MODESW'] },
-    { bits: 2, name: 0x0, attr: ['2', 'FUNCT2'] },
+    { bits: 2, name: 0x3, attr: ['2', 'FUNCT2'] },
     { bits: 3, name: 0x0, attr: ['3', 'FUNCT3'] },
     { bits: 3, name: 0x7, attr: ['3', 'FUNCT3'] },
     { bits: 3, name: 0x4, attr: ['3', 'FUNCT3'] },