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 <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/process/backporting.rst

···
 ==============================
 
 As the stable maintainers try to cherry-pick mainline fixes onto their
-stable kernels, they may send out emails asking for backports when when
+stable kernels, they may send out emails asking for backports when
 encountering conflicts, see e.g.
 <https://lore.kernel.org/stable/2023101528-jawed-shelving-071a@gregkh/>.
 These emails typically include the exact steps you need to cherry-pick
···
 format::
 
   <original patch title>
-  
+
   [ Upstream commit <mainline rev> ]
-  
+
   <rest of the original changelog>
   [ <summary of the conflicts and their resolutions> ]
   Signed-off-by: <your name and email>