Minor: add string quotes in command

Author: faze-geek

README.md

@@ -40,7 +40,7 @@ source activate  "xeus-cpp"
 ```
 We will now install the dependencies needed to compile xeux-cpp from source within this environment by executing the following
 ```bash
-mamba install notebook cmake cxx-compiler xeus-zmq nlohmann_json=3.11.3 jupyterlab CppInterOp cpp-argparse<3.1 pugixml doctest -c conda-forge
+mamba install notebook cmake cxx-compiler xeus-zmq nlohmann_json=3.11.3 jupyterlab CppInterOp cpp-argparse"<3.1" pugixml doctest -c conda-forge
 ```
 Now you can compile the kernel from the source by executing (replace `$CONDA_PREFIX` with a custom installation prefix if need be)
 ```bash