docs: fix typos and whitespace in Documentation/process/backporting.rst

- Fix repeated word "when" in backporting documentation
- Remove trailing whitespace after '$' character

These issues were reported by checkpatch.pl. No functional changes.

Signed-off-by: Abhinav Saxena <xandfury@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241107061911.106040-1-xandfury@gmail.com