Fix typo.

Co-authored-by: Oleksandr Pavlyk <[email protected]>

Author: alliepiper

ci/matrix.yaml

@@ -22,7 +22,7 @@ llvm16: &llvm16 { name: 'llvm', version: '16', exe: 'clang++' }
 llvm17: &llvm17 { name: 'llvm', version: '17', exe: 'clang++' }
 llvm18: &llvm18 { name: 'llvm', version: '18', exe: 'clang++' }
 llvm19: &llvm19 { name: 'llvm', version: '19', exe: 'clang++' }
-llvm19: &llvm20 { name: 'llvm', version: '20', exe: 'clang++' }
+llvm20: &llvm20 { name: 'llvm', version: '20', exe: 'clang++' }
 
 # Each environment below will generate a unique build/test job
 # See the "compute-matrix" job in the workflow for how this is parsed and used