Check notebook chunk

[sjspielman]

There is something that needs to be fixed with this chunk, but I only wrote down in my notes was "check plot_marker_expression chunk" in my notes, and I don't precisely remember what I meant! But, @jashapiro should know.

training-modules/scRNA-seq/05-clustering_markers_scRNA.Rmd

Lines 320 to 331 in 624613d

	```{r plot_marker_expression, live = TRUE}
	# get gene id and gene symbol for nicer plotting
	rank <- 1
	gene_id <- gene_info$ID[1]
	symbol <- gene_info$Symbol[1]
	# Plot UMAP results colored by expression
	plotReducedDim(hodgkins_sce, "UMAP",
	colour_by = gene_id) +
	# label the guide with the gene symbol
	guides(color = guide_colourbar(title = symbol))
	```

Noting also this is the chunk where we saw, as mentioned in #550, that we don't want to be labeling UMAP legends with labs().

[jashapiro]

Thank you for noting this! The error is that the gene_id and symbol definitions should use rank rather than hard coding!