Update vignette to install SeuratData

Author: agdenadel

vignettes/basic-usage.Rmd

@@ -18,12 +18,15 @@ knitr::opts_chunk$set(
 ```{r setup}
 suppressPackageStartupMessages({
 library(Seurat)
-library(SeuratData)
-
 library(callback)
 })
 ```
 
+```{r install_SeuratData}
+devtools::install_github('satijalab/seurat-data')
+library(SeuratData)
+```
+
 First, we use the `SeuratData` data package to first download and then load
 2700 PBMCs. The loaded `SeuratObject`, `pbmc3k`, is from an old version of 
 `Seurat`, and so we update the object to v5.