Update ch8 1thread.rst

Author: lewiszlw

source/chapter8/1thread.rst

@@ -235,7 +235,7 @@
 
 	            available.ctx.x1 = guard as u64;  //ctx.x1  is old return address
 	            available.ctx.nx1 = f as u64;     //ctx.nx1 is new return address
-	            available.ctx.x2 = s_ptr.offset(32) as u64; //cxt.x2 is sp
+	            available.ctx.x2 = s_ptr.offset(-32) as u64; //cxt.x2 is sp
 
 	        }
 	        available.state = State::Ready;