Update Singularity build command in installation.md

[kmlhpk]

Just a small change to the Singularity section of the install instructions:

docker://multiqc/multiqc:1.20 is not the right URI, need to use v1.20 (replacing version as appropriate) as the tag instead

Using the former command results in FATAL: While performing build: conveyor failed to get: reading manifest 1.20 in docker.io/multiqc/multiqc: manifest unknown: manifest unknown

Using v1.19 instead worked.

• This comment contains a description of changes (with reason)

[ewels]

Great spot, thank you! I noticed another instance of the same typo further up too, for Docker. So fixed that and pushed to your PR..

[kmlhpk]

cool, thanks! happy to have helped :)