fix typo in doc (#3436)

docs/developer-guide/low-level-operation-api.md

@@ -116,7 +116,7 @@ void transpose(const ncnn::Mat& in, ncnn::Mat& out)
         if (dims == 3) elemcount = in.elempack * in.c;
 
         int dst_elempack = 1;
-        if (layer->support_packing)
+        if (op->support_packing)
         {
             if (elemcount % 8 == 0 && (ncnn::cpu_support_x86_avx2() || ncnn::cpu_support_x86_avx()))
                 dst_elempack = 8;
@@ -193,7 +193,7 @@ void normalize(const ncnn::Mat& in, ncnn::Mat& out)
         if (dims == 3) elemcount = in_fp16.elempack * in_fp16.c;
 
         int dst_elempack = 1;
-        if (layer->support_packing)
+        if (op->support_packing)
         {
             if (elemcount % 8 == 0 && (ncnn::cpu_support_x86_avx2() || ncnn::cpu_support_x86_avx()))
                 dst_elempack = 8;