remove mention of GRCh37 from template if igenomes is skipped

closes nf-core#3322

nf_core/pipeline-template/docs/usage.md

@@ -61,7 +61,7 @@ An [example samplesheet](../assets/samplesheet.csv) has been provided with the p
 The typical command for running the pipeline is as follows:
 
 ```bash
-nextflow run {{ name }} --input ./samplesheet.csv --outdir ./results --genome GRCh37 -profile docker
+nextflow run {{ name }} --input ./samplesheet.csv --outdir ./results {% if igenomes %}--genome GRCh37{% endif %} -profile docker
 ```
 
 This will launch the pipeline with the `docker` configuration profile. See below for more information about profiles.
@@ -93,7 +93,9 @@ with:
 ```yaml title="params.yaml"
 input: './samplesheet.csv'
 outdir: './results/'
+{% if igenomes -%}
 genome: 'GRCh37'
+{% endif -%}
 <...>
 ```