fix syntax error in suggestion

Author: AA-Turner

Tools/build/compute-changes.py

@@ -18,7 +18,7 @@
 if TYPE_CHECKING:
     from collections.abc import Set
 
-GITHUB_DEFAULT_BRANCH = os.environ["GITHUB_DEFAULT_BRANCH]
+GITHUB_DEFAULT_BRANCH = os.environ["GITHUB_DEFAULT_BRANCH"]
 GITHUB_CODEOWNERS_PATH = Path(".github/CODEOWNERS")
 GITHUB_WORKFLOWS_PATH = Path(".github/workflows")
 CONFIGURATION_FILE_NAMES = frozenset({