Normalize pip-tools' header comment @ `runtime.txt`

It's currently not prefixed with `requirements/` in most places and
that what Dependabot keeps using.

requirements/runtime.txt

@@ -2,7 +2,7 @@
 # This file is autogenerated by pip-compile with Python 3.11
 # by the following command:
 #
-#    pip-compile --allow-unsafe --output-file=requirements/runtime.txt --resolver=backtracking --strip-extras requirements/runtime.in
+#    pip-compile --allow-unsafe --output-file=runtime.txt --resolver=backtracking --strip-extras runtime.in
 #
 annotated-types==0.5.0
     # via pydantic