Improve table: Compressed instruction formats (#1696)

Table "Compressed 16-bit RVC instruction formats" header indicating the bits corresponding to each column only aligns properly with the first row (CR format), but doesn't reflect the boundaries of other rows (for example, funct3 or rs1'), making it hard to read.

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

src/c-st-ext.adoc

@@ -243,7 +243,7 @@ encoding space for other instructions requiring fewer operand bits.
 |
 [float="left",align="left",cols="1,1,1,1,1,1,1",options="noheader"]
 !===
-2+^!15 14 13 12 2+^!11 10 9 8 7 2+^!6 5 4 3 2 ^!1 0
+^!15 14 13 ^!12 ^!11 10 ^!9 8 7 ^!6 5 ^!4 3 2 ^!1 0
 2+^!funct4 2+^!rd/rs1 2+^!rs2 ^!  op
 ^!funct3 ^!imm 2+^!rd/rs1  2+^!imm ^!  op
 ^!funct3 3+^!imm  2+^!rs2 ^!  op