corrected --matrix to --imatrix

README.md

@@ -119,7 +119,7 @@ cd <llama.cpp directory>
 ```
 4. Use the generated matrix file to quantise the model (see further down for some pre-computed matrix files)
 ```
-./quantize --matrix <output.matrix> <model_path>/ggml-model-f16.gguf <quantisation_level, eg:IQ4_XS>
+./quantize --imatrix <output.matrix> <model_path>/ggml-model-f16.gguf <quantisation_level, eg:IQ4_XS>
 ```
 Note: normal quantisation also benefits from using a matrix file. It also seem that a bigger input matrix is
 better for higher quantisation.