Fix indentation in containerize example (#17228)

[george.hartzell@172-16-193-97 spack-explore-docker]$ spack containerize
Running `spack containerize` with the example `spack.yaml` file fails
with an error that ends like so:

```
[...]
  File "/local_scratch/hartzell/tmp/spack-explore-docker/lib/spack/external/ruamel/yaml/scanner.py", line 165, in need_more_tokens
    self.stale_possible_simple_keys()
  File "/local_scratch/hartzell/tmp/spack-explore-docker/lib/spack/external/ruamel/yaml/scanner.py", line 309, in stale_possible_simple_keys
    "could not find expected ':'", self.get_mark())
ruamel.yaml.scanner.ScannerError: while scanning a simple key
  in "/local_scratch/hartzell/tmp/spack-explore-docker/spack.yaml", line 26, column 1
could not find expected ':'
  in "/local_scratch/hartzell/tmp/spack-explore-docker/spack.yaml", line 28, column 5
```

Indenting the block string fixes the problem for me.

CentOS 7,

```
$ spack --version
0.14.2-1529-ec58f28c2
```
This commit is contained in:
George Hartzell 2020-06-28 23:58:51 -07:00 committed by GitHub
parent d27ea3be1c
commit e6bb8360d0
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@ -165,7 +165,7 @@ of environments:
# Extra instructions
extra_instructions:
final: |
RUN echo 'export PS1="\[$(tput bold)\]\[$(tput setaf 1)\][gromacs]\[$(tput setaf 2)\]\u\[$(tput sgr0)\]:\w $ \[$(tput sgr0)\]"' >> ~/.bashrc
RUN echo 'export PS1="\[$(tput bold)\]\[$(tput setaf 1)\][gromacs]\[$(tput setaf 2)\]\u\[$(tput sgr0)\]:\w $ \[$(tput sgr0)\]"' >> ~/.bashrc
# Labels for the image
labels: