Fix the release label

Author: alexreinking

master/master.cfg

@@ -116,7 +116,7 @@ LLVM_BRANCHES = {
 # (Note that there are older releases of Halide that we no longer bother to build/test regularly.)
 
 HALIDE_MAIN = "main"
-HALIDE_RELEASE_21 = "release_19"
+HALIDE_RELEASE_21 = "release_21"
 HALIDE_RELEASE_19 = "release_19"
 # HALIDE_RELEASE_18 = "release_18"
 # HALIDE_RELEASE_17 = "release_17"