Fix annotation context (#1630)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 973afe5..0f0adbc 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2242,7 +2242,8 @@
                 "buildkite-agent",
                 "annotate",
                 "--style=info",
-                "--context 'diff'",
+                "--context",
+                "'diff'",
                 "This run only contains test targets that have been changed since "
                 "{} due to the {} env variable".format(resolved_diffbase, USE_BAZEL_DIFF_ENV_VAR),
             ]