README.md: fix a broken test

Spurious "&mut" is a duplicate, "mbuff" has already been declared as
"mut" before. Remove it to fix test compilation.

README.md

@@ -328,7 +328,7 @@ fn main() {
 
     // Here we must provide both a reference to the packet data, and to the
     // metadata buffer we use.
-    unsafe { assert_eq!(vm.prog_exec_jit(mem, &mut mbuff), 0x2211); }
+    unsafe { assert_eq!(vm.prog_exec_jit(mem, mbuff), 0x2211); }
 }
 ```