Fix small typo in 03 notebook

Really small, but worth the change.

scRNA-seq-advanced/03-differential_expression.Rmd

@@ -26,7 +26,7 @@ In this notebook, we will work with multiple samples to identify differentially
 ![Single-cell roadmap: Differential expression](diagrams/roadmap_differential_expression.png)
 
 We will continue working with samples from the [`SCPCP000005` project](https://scpca.alexslemonade.org/projects/SCPCP000005), an investigation of pediatric solid tumors led by the Dyer and Chen labs at St. Jude Children's Research Hospital.
-This particular dataset contains 10 different samples that have been integrated using `fastMNN`, following the same procedure we outlined in `03-dataset_integration.Rmd`.
+This particular dataset contains 10 different samples that have been integrated using `fastMNN`, following the same procedure we outlined in `02-dataset_integration.Rmd`.
 These 10 samples represent two different types of rhabdomyosarcoma (RMS): embryonal rhabdomyosarcoma (ERMS) and alveolar rhabdomyosarcoma (ARMS).
 These two subtypes are distinguished by the presence of the `PAX3/PAX7-FOXO1` fusion gene, which is present only in ARMS patients.
 Additionally, cells found in ARMS tumors tend to have an increased mutational burden with cells in a more differentiated state compared to ERMS tumor cells ([Shern _et al._ 2014](https://doi.org/10.1158/2159-8290.CD-13-0639); [Stewart _et al._ 2018](https://doi.org/10.1016/j.ccell.2018.07.012)).