fix: forgot a line under compiling for linux

opening-hdf5/README.md

@@ -20,16 +20,20 @@ make
 ```
 
 ### Building with Linux
-Makes use of `mdspan` which is not supported by GCC libstdc++ at time of writing. See [compiler support](https://en.cppreference.com/w/cpp/compiler_support/23) for `mdspan`. The solution to this is to use Clang and libc++; this is configured in our CMake setup, however the default installation of the `netcdf-cxx` package on at least Arch linux (and suspectedly Debian derivatves as well) specifically builds for the glibc implementation. To get the netcdf C++ bindings functional with the libc++ implementation, one needs to build from source. On Linux, this requires a few changes to the CMake file included with the netcdf-cxx source code, which are detailed below.
+Makes use of `mdspan` which is not supported by glibc++ at time of writing. See [compiler support](https://en.cppreference.com/w/cpp/compiler_support/23) for `mdspan`. The solution to this is to use Clang and libc++; this is configured in our CMake setup, however the default installation of the `netcdf-cxx` package on at least Arch linux (and suspectedly Debian derivatives as well) specifically builds for the glibc implementation. To get the netcdf C++ bindings functional with the libc++ implementation, one needs to build from source. On Linux, this requires a few changes to the CMake file included with the netcdf-cxx source code, which are detailed below.
 
 Step-by-step to build the program using clang++ and libc++ on linux:
  1. Download the source code of netcdf-cxx, found at 'https://github.com/Unidata/netcdf-cxx4/releases/tag/v4.3.1' (make sure to download the release source code, as the master branch contains non-compilable code).
- 2. Build the source code with the following flags: 
+ 2. Edit the CMakeLists.txt file, by appending '-stdlib=libc++' to the `CMAKE_CXX_FLAGS` variable in line 430. This means line 430 should read: 
+    ```cmake 
+    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -Wall -Wno-unused-variable -Wno-unused-parameter -stdlib=libc++")
+    ```
+ 2. Build the source code with the following: 
   ```sh 
     mkdir build && cd build
     cmake .. -DCMAKE_CXX_COMPILER=/usr/bin/clang++
     make
     ctest
-    make install
+    sudo make install
   ```
  3. Now the code should compile through the standard steps described in the Compiling section.