It would be nice to integrate salmon tutorial dataset with the tximport example as salmon tutorial is on A.thaliana while tx2gene function for tximport uses gencode which is for human genome.

How do I do it for A.thaliana?

The tx2gene argument is for you to make. There are many ways to make this table using Bioconductor, but the key is that you must use the same source as you used for downloading the transcript sequences.