Wording improvement in GWAS notebook

Author: tomwhite

docs/examples/gwas_tutorial.ipynb

@@ -117,7 +117,7 @@
    "id": "plastic-running",
    "metadata": {},
    "source": [
-    "We passed a few arguments to the `vcf_to_zarr` conversion function, so it only loads the first alternate allele (`max_alt_alleles=1`), and to load extra VCF fields we are interested in (`GT`, `DP`, `GQ`, and `AD`). Also, `AD` needed defining as having a `Number` definition of `R` (one value for each allele, including the reference), since the dataset we are using defines it as `.` which means \"unknown\".\n",
+    "We passed a few arguments to the `vcf_to_zarr` conversion function, so it only converts the first alternate allele (`max_alt_alleles=1`), and to load extra VCF fields we are interested in (`GT`, `DP`, `GQ`, and `AD`). Also, `AD` needed defining as having a `Number` definition of `R` (one value for each allele, including the reference), since the dataset we are using defines it as `.` which means \"unknown\".\n",
     "\n",
     "Now the data has been written as Zarr, all downstream operations on will be much faster. Note that sgkit uses an [Xarray](http://xarray.pydata.org/en/stable/) dataset to represent the VCF data, where Hail uses MatrixTable."
    ]