cmd/internal/obj/arm64: fix typos in document

The current document mismatches Go syntax loads a signed-byte
instruction "MOVB" with GNU syntax loads an 64bit double-word
instruction "ldr". This is just a typo in the document, the
assembler has the correct encoding. This patch fix this error.

Fixes #39367

Change-Id: Idb8f65ca540514ee5bc8f07073e756838710ba93
Reviewed-on: https://go-review.googlesource.com/c/go/+/236217
Reviewed-by: Cherry Zhang <cherryyz@google.com>
diff --git a/src/cmd/internal/obj/arm64/doc.go b/src/cmd/internal/obj/arm64/doc.go
index d66e6f8..75152175 100644
--- a/src/cmd/internal/obj/arm64/doc.go
+++ b/src/cmd/internal/obj/arm64/doc.go
@@ -22,7 +22,8 @@
 
   Examples:
     MOVD.P -8(R10), R8         <=>      ldr x8, [x10],#-8
-    MOVB.W 16(R16), R10        <=>      ldr x10, [x16,#16]!
+    MOVB.W 16(R16), R10        <=>      ldrsb x10, [x16,#16]!
+    MOVBU.W 16(R16), R10       <=>      ldrb x10, [x16,#16]!
 
 3. Go uses a series of MOV instructions as load and store.