fix: indent in config section code block

[cmelone]

One of the code blocks in the config section was missing a space in front of curl, leading to yaml parsing errors like:

==> Error: Invalid environment configuration detected: error parsing YAML: near /home/spack7/spack/var/spack/environments/config-env/spack.yaml, 8, 6: expected <block end>, but found '?' in /home/spack7/spack/var/spack/environments/config-env/spack.yaml