Version bump in README.

Author: lima1

README.md

@@ -31,7 +31,7 @@ For outdated R/Bioconductor versions, you can try backporting the latest stable
 version (this should work fine for Bioconductor 3.3 and later):
 
 ```
-BiocManager::install("lima1/PureCN", ref = "RELEASE_3_20")
+BiocManager::install("lima1/PureCN", ref = "RELEASE_3_21")
 ```
 
 If you want the latest and greatest from the developer branch: