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 <[e-mail]>
Signed-off-by: Marko Turk <[e-mail]>
Reviewed-by: Dirk Behme <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Danilo Krummrich <[e-mail]>

rust/kernel/pci/io.rs

···
 ///
 /// # Invariants
 ///
-/// `Bar` always holds an `IoRaw` inststance that holds a valid pointer to the start of the I/O
+/// `Bar` always holds an `IoRaw` instance that holds a valid pointer to the start of the I/O
 /// memory mapped PCI BAR and its size.
 pub struct Bar<const SIZE: usize = 0> {
     pdev: ARef<Device>,
···
         let ioptr: usize = unsafe { bindings::pci_iomap(pdev.as_raw(), num, 0) } as usize;
         if ioptr == 0 {
             // SAFETY:
-            // `pdev` valid by the invariants of `Device`.
+            // `pdev` is valid by the invariants of `Device`.
             // `num` is checked for validity by a previous call to `Device::resource_len`.
             unsafe { bindings::pci_release_region(pdev.as_raw(), num) };
             return Err(ENOMEM);