Use explicit Arc::clone instead of calling .clone() method (#596)

This is to highlight that the clones are cheap: they run custom logic
in the reference counted type, they don’t do a deep clone.

src/concurrency/shared_state/arc.md

@@ -1,6 +1,6 @@
 # `Arc`
 
-[`Arc<T>`][1] allows shared read-only access via its `clone` method:
+[`Arc<T>`][1] allows shared read-only access via `Arc::clone`:
 
 ```rust,editable
 use std::thread;
@@ -10,7 +10,7 @@ fn main() {
     let v = Arc::new(vec![10, 20, 30]);
     let mut handles = Vec::new();
     for _ in 1..5 {
-        let v = v.clone();
+        let v = Arc::clone(&v);
         handles.push(thread::spawn(move || {
             let thread_id = thread::current().id();
             println!("{thread_id:?}: {v:?}");

src/concurrency/shared_state/example.md

@@ -7,7 +7,7 @@ use std::thread;
 // use std::sync::{Arc, Mutex};
 
 fn main() {
-    let mut v = vec![10, 20, 30];
+    let v = vec![10, 20, 30];
     let handle = thread::spawn(|| {
         v.push(10);
     });
@@ -29,7 +29,7 @@ use std::thread;
 fn main() {
     let v = Arc::new(Mutex::new(vec![10, 20, 30]));
 
-    let v2 = v.clone();
+    let v2 = Arc::clone(&v);
     let handle = thread::spawn(move || {
         let mut v2 = v2.lock().unwrap();
         v2.push(10);