fix: install instructions in README

- adjust git clone command to clone correct spack version
- remove creation of enviroement (already present due to `spack.yaml`)
- provide a example for `SYSTEMNAME`

README.md

@@ -40,7 +40,7 @@ git clone --branch {branch-name} https://gitlab.ebrains.eu/ri/tech-hub/platform/
 
 Clone Spack. We currently use version v0.21.1:
 ```
-git clone --depth 1 -c advice.detachedHead=false -c feature.manyFiles=true --branch v0.20.0 https://github.com/spack/spack
+git clone --depth 1 -c advice.detachedHead=false -c feature.manyFiles=true --branch v0.21.1 https://github.com/spack/spack
 ```
 
 Activate Spack:
@@ -53,12 +53,7 @@ Add the project repository to your Spack environment:
 spack repo add ebrains-spack-builds
 ```
 
-Create the environment:
-```
-spack env create -d ebrains-spack-builds/
-```
-
-Define your site-specific configurations:
+Define your site-specific configurations, e.g. choose `ebrainslab`:
 ```
 export SYSTEMNAME=<your-system-name>
 mkdir ebrains-spack-builds/site-config/$SYSTEMNAME