Improve Cross-Platform Build Instructions in Documentation (#3229)

* Update build

* Simplified build command length for cross-platform compatibility.

* Modified to replace `make -j` only with `cmake --build.`

README.md

@@ -6,7 +6,7 @@ Very fast, header-only/compiled, C++ logging library. [![ci](https://github.com/
 ```console
 $ git clone https://github.com/gabime/spdlog.git
 $ cd spdlog && mkdir build && cd build
-$ cmake .. && make -j
+$ cmake .. && cmake --build .
 ```
 
 see example [CMakeLists.txt](https://github.com/gabime/spdlog/blob/v2.x/example/CMakeLists.txt) on how to use.