docs: Avoid using get(0) in examples

I used this example to implement tests, but clippy is not much happy
about that so changing also the docs to avoid leading by bad example
next fellow readers:

error: accessing first element with `attrs.get(0)`
    --> cryptoki/tests/basic.rs:2528:26
     |
2528 |         assert!(matches!(attrs.get(0), Some(Attribute::UniqueId(_))));
     |                          ^^^^^^^^^^^^ help: try: `attrs.first()`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first

Signed-off-by: Jakub Jelen <jjelen@redhat.com>

Author: Jakuje

cryptoki/src/session/object_management.rs

@@ -57,7 +57,7 @@ const MAX_OBJECT_COUNT: NonZeroUsize = unsafe { NonZeroUsize::new_unchecked(10)
 ///
 ///     let attributes = session.get_attributes(obj, &wanted_attr)?;
 ///
-///     match attributes.get(0) {
+///     match attributes.first() {
 ///         Some(Attribute::Label(l)) => {
 ///             println!(
 ///                 "token object #{}: handle {}, label {}",
@@ -413,7 +413,7 @@ impl Session {
     /// session.login(UserType::User, Some(&AuthPin::new("fedcba".into())));
     ///
     /// let empty_attrib= vec![];
-    /// if let Some(object) = session.find_objects(&empty_attrib).unwrap().get(0) {
+    /// if let Some(object) = session.find_objects(&empty_attrib).unwrap().first() {
     ///     let attribute_types = vec![
     ///         AttributeType::Token,
     ///         AttributeType::Private,