modify format information of ref.txt

README.md

@@ -76,7 +76,7 @@ CIRCexplorer.py -f tophat_fusion/accepted_hits.bam -g hg19.fa -r ref.txt
 
 ###Note
 
-* ref.txt is in the format (refFlat.txt in UCSC) below (see details in [the example file](https://github.com/YangLab/CIRCexplorer/blob/master/example/ref_example.txt))
+* ref.txt is in the format ([Gene Predictions and RefSeq Genes with Gene Names](https://genome.ucsc.edu/FAQ/FAQformat.html#format9)) below (see details in [the example file](https://github.com/YangLab/CIRCexplorer/blob/master/example/ref_example.txt))
 
 | Field       | Description                   |
 | :---------: | :---------------------------- |