Fix alleles dimension in GWAS tutorial schema

Author: tomwhite

docs/examples/1kg.schema.json

@@ -209,7 +209,7 @@
             "dimensions": [
                 "variants",
                 "samples",
-                "FORMAT_AD_dim"
+                "alleles"
             ],
             "description": "",
             "vcf_field": "FORMAT/AD",