fix single usage of implicit input in closure

Author: pinin4fjords

docs/hello_nextflow/05_hello_operators.md

@@ -600,7 +600,7 @@ _After:_
 
 ```groovy title="hello-operators.nf" linenums="87"
     script:
-    def gvcfs_line = all_gvcfs.collect { "-V ${it}" }.join(' ')
+    def gvcfs_line = all_gvcfs.collect { gvcf -> "-V ${gvcf}" }.join(' ')
     """
     gatk GenomicsDBImport \
         ${gvcfs_line} \