Update UMAP_plot_wrapper() function

[sjspielman]

The sce argument needs to be documented in this function:

training-modules/scRNA-seq/04-dimension_reduction_scRNA.Rmd

Lines 398 to 412 in 624613d

	```{r UMAP-function}
	UMAP_plot_wrapper <- function(sce = normalized_sce, nn_param = 15) {
	# Purpose: Run UMAP and plot the output
	# Args: nn_param: a single numeric argument that will change the
	# n_neighbors variable in the calculateUMAP() function.
	# Output: a scatterplot with the two UMAP coordinates plotted and
	# cell-types labeled with data point colors.
	# Run UMAP with a specified n_neighbors parameter
	sce_umap <- scater::runUMAP(sce, dimred = "PCA", n_neighbors = nn_param)
	scater::plotReducedDim(sce_umap, "UMAP", colour_by = "detected") +
	# make the legend label more informative (this is ggplot2 code!)
	guides(color = guide_colourbar(title="genes\nexpressed"))
	}
	```

Noting also that during training, @jashapiro wondered if we should change guides() to labs(colour = ...), but then a bit later during training when we made another UMAP plot, that approach didn't play well with scater's function, so sticking with guides() here is probably the thing to do.

[jashapiro]

Thank you for filing this!