rust/pv: Fix some comments

The library is used by multiple PV related tools, not only for managing
the guest secret store.
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>

rust/pv_core/src/confidential.rs

@@ -90,7 +90,7 @@ impl<C: Zeroize> Confidential<C> {
         &self.0
     }
 
-    /// Get an immutable reference to the contained value
+    /// Get a mutable reference to the contained value
     ///
     /// NOTE that modifications to a mutable reference can trigger reallocation.
     /// e.g. a [`Vec`] might expand if more space needed. -> preallocate enough space