[Doc] Fix errors in SIL.rst. (#27476)
Fix "Title underline too short" errors.
Sphinx has been installed on TensorFlow Ubuntu CI machines to catch doc errors.
Sphinx is not installed on the macOS CI machine because doc building fails:
`Pygments lexer name 'sil' is not known`.
* Fix difficult-to-reproduce failure due to `-dump-ast 2>&1`.
This failure occurred only on the macOS CI machine: reproducing locally did not
work. It may have been introduced in https://github.com/apple/swift/pull/27471
but leaked through because the macOS CI machine was down at the time of testing.
