runtime: update documentation for ios addr bits

After this merge: https://go-review.googlesource.com/c/go/+/344401, ios/arm64 was treated as a 64 bit system and the addr bits of ios/arm64 was set to 40

Change-Id: I32d72787d20a3cf952b036e3e887cf5bae2273d8
GitHub-Last-Rev: 8917029
GitHub-Pull-Request: #69343
Reviewed-on: https://go-review.googlesource.com/c/go/+/610856
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Tim King <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: WuGuangyao

src/runtime/malloc.go

@@ -227,7 +227,7 @@ const (
 	// --------------  ---------  ----------  ----------  -----------
 	//       */64-bit         48        64MB           1    4M (32MB)
 	// windows/64-bit         48         4MB          64    1M  (8MB)
-	//      ios/arm64         33         4MB           1  2048  (8KB)
+	//      ios/arm64         40         4MB           1  256K  (2MB)
 	//       */32-bit         32         4MB           1  1024  (4KB)
 	//     */mips(le)         31         4MB           1   512  (2KB)