rust: pci: fix typos in Bar struct's comments

rust: pci: fix typos in Bar struct's comments

Fix a typo in the doc-comment of the Bar structure: 'inststance ->
instance'.

Add also 'is' to the comment inside Bar's `new()` function (suggested
by Dirk):
// `pdev` is valid by the invariants of `Device`.

Fixes: bf9651f84b4e ("rust: pci: implement I/O mappable `pci::Bar`")
Suggested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Marko Turk <mt@markoturk.info>
Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://patch.msgid.link/20260105213726.73000-2-mt@markoturk.info
Signed-off-by: Danilo Krummrich <dakr@kernel.org>