Fix range representation for C.ADDI16SP immediate (#1634)

It's an inclusive range, and it should have a space after the comma.

src/c-st-ext.adoc

@@ -609,7 +609,7 @@ C.ADDI16SP (add immediate to stack pointer)
 shares the opcode with C.LUI, but has a destination field of
 `x2`. C.ADDI16SP adds the non-zero sign-extended 6-bit immediate to the
 value in the stack pointer (`sp=x2`), where the immediate is scaled to
-represent multiples of 16 in the range (-512,496). C.ADDI16SP is used to
+represent multiples of 16 in the range [-512, 496]. C.ADDI16SP is used to
 adjust the stack pointer in procedure prologues and epilogues. It
 expands into `addi x2, x2, nzimm[9:4]`. C.ADDI16SP is only valid when
 _nzimm_≠0; the code point with _nzimm_=0 is reserved.