small fixes on option message

Author: Qianqian Zhu

src/command/marian_conv.cpp

@@ -18,7 +18,8 @@ int main(int argc, char** argv) {
     YAML::Node config; // @TODO: get rid of YAML::Node here entirely to avoid the pattern. Currently not fixing as it requires more changes to the Options object.
     auto cli = New<cli::CLIWrapper>(
         config,
-        "Convert a model in the .npz format and normal memory layout to a mmap-able binary model which could be in normal memory layout or packed memory layout",
+        "Convert a model in the .npz format and normal memory layout to a mmap-able binary model which could be in normal memory layout or packed memory layout\n"
+        "or convert a text lexical shortlist to a binary shortlist with {--shortlist,-s} option",
         "Allowed options",
         "Examples:\n"
         "  ./marian-conv -f model.npz -t model.bin --gemm-type packed16");