[Github] Switch vectorization PR label to vectorizers (#111633)

This changes the PR label to match the name of the subscriber team.

Fixes #111485.

.github/new-prs-labeler.yml

@@ -69,7 +69,7 @@ PGO:
   - llvm/**/llvm-profdata/**/*
   - llvm/**/llvm-profgen/**/*
 
-vectorization:
+vectorizers:
   - llvm/lib/Transforms/Vectorize/**/*
   - llvm/include/llvm/Transforms/Vectorize/**/*