Added comma to improve readability (#312)

jupyter-book/cellular_structure/annotation.ipynb

@@ -180,7 +180,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "In this setting the data is often clustered before annotation, so that we can annotate groups of cells instead of making a per-cell call. This is not only less laborious, but also more robust to noise: a single cell might not have a count for a specific marker even if it was expressed in that cell, simply due to the inherent sparsity of single cell data. Clustering enables the detection of cells highly similar in overall gene expression and can therefore account for drop-outs at single cell level. "
+    "In this setting, the data is often clustered before annotation, so that we can annotate groups of cells instead of making a per-cell call. This is not only less laborious, but also more robust to noise: a single cell might not have a count for a specific marker even if it was expressed in that cell, simply due to the inherent sparsity of single cell data. Clustering enables the detection of cells highly similar in overall gene expression and can therefore account for drop-outs at single cell level. "
    ]
   },
   {