Add --output-format to usage

Author: EAddario

tools/imatrix/README.md

@@ -10,7 +10,7 @@ More information is available in <https://github.com/ggml-org/llama.cpp/pull/486
     -m model.gguf -f some-text.txt [-o imatrix.gguf] [--output-format {gguf,dat}] [--no-ppl] \
     [--process-output] [--chunk 123] [--save-frequency 0] [--output-frequency 10] \
     [--in-file imatrix-prev-0.gguf --in-file imatrix-prev-1.gguf ...] [--parse-special] \
-    [--activation-statistics] [--show-statistics] [...]
+    [--output-format gguf|dat] [--activation-statistics] [--show-statistics] [...]
 ```
 
 Here `-m | --model` with a model name and `-f | --file` with a file containing calibration data (such as e.g. `wiki.train.raw`) are mandatory.