fix ncnn2table specification (#3006)

docs/how-to-use-and-FAQ/quantized-int8-inference.md

@@ -24,7 +24,7 @@ find images/ -type f > imagelist.txt
 ```
 
 * mean and norm are the values you passed to ```Mat::substract_mean_normalize()```
-* shape is the blob shape of your model
+* shape is the blob shape of your model, [w,h] or [w,h,c]
 * pixel is the pixel format of your model, image pixels will be converted to this type before ```Extractor::input()```
 * thread is the CPU thread count that could be used for parallel inference
 * method is the post training quantization algorithm, kl and aciq are currently supported

tools/quantize/ncnn2table.cpp

@@ -1571,7 +1571,7 @@ static void show_usage()
     fprintf(stderr, "Usage: ncnn2table [ncnnparam] [ncnnbin] [list,...] [ncnntable] [(key=value)...]\n");
     fprintf(stderr, "  mean=[104.0,117.0,123.0],...\n");
     fprintf(stderr, "  norm=[1.0,1.0,1.0],...\n");
-    fprintf(stderr, "  shape=[224,224,3],...\n");
+    fprintf(stderr, "  shape=[224,224,3],...[w,h,c] or [w,h]\n");
     fprintf(stderr, "  pixel=RAW/RGB/BGR/GRAY/RGBA/BGRA,...\n");
     fprintf(stderr, "  thread=8\n");
     fprintf(stderr, "  method=kl/aciq/eq\n");