update readme

README.md

@@ -18,9 +18,11 @@ for deriving the inferred haplotype sequence against the list of SNP markers.
 ## Usage
 Obtain allele counts for cell barcodes listed in barcodeFile at the SNP positions in VCF from the BAM file.
 
+```
 Usage:
     sscocaller [options] <BAM> <VCF> <barcodeFile> <out_prefix>
 
+
 Options:
     -t --threads <threads> number of BAM decompression threads [default: 4]
     -MQ --minMAPQ <mapq> Minimum MAPQ for read filtering [default: 20]
@@ -36,4 +38,4 @@ Options:
 
 Examples
     ./sscocaller --threads 10 AAAGTAGCACGTCTCT-1.raw.bam AAAGTAGCACGTCTCT-1.raw.bam.dp3.alt.vcf.gz barcodeFile.tsv ./percell/ccsnp-
-
+```