cmd/compile: fix typos in document

Correct "a2Spill" to "a3Spill"

Change-Id: I6ac4c45973dfaeb16d3a90d835589b6af1aefe1d
Reviewed-on: https://go-review.googlesource.com/c/go/+/331850
Trust: fannie zhang <Fannie.Zhang@arm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Author: zhangfannie

src/cmd/compile/abi-internal.md

@@ -233,7 +233,7 @@ stack frame is laid out in the following sequence:
     r1.x    uintptr
     r1.y    [2]uintptr
     a1Spill uint8
-    a2Spill uint8
+    a3Spill uint8
     _       [6]uint8  // alignment padding
 
 In the stack frame, only the `a2` field is initialized on entry; the