Fix UB in as_ptr_cast_mut documentation

Author: Alexendoo

clippy_lints/src/casts/mod.rs

@@ -625,14 +625,14 @@ declare_clippy_lint! {
     ///
     /// ### Example
     /// ```rust
-    /// let string = String::with_capacity(1);
-    /// let ptr = string.as_ptr() as *mut u8;
+    /// let mut vec = Vec::<u8>::with_capacity(1);
+    /// let ptr = vec.as_ptr() as *mut u8;
     /// unsafe { ptr.write(4) }; // UNDEFINED BEHAVIOUR
     /// ```
     /// Use instead:
     /// ```rust
-    /// let mut string = String::with_capacity(1);
-    /// let ptr = string.as_mut_ptr();
+    /// let mut vec = Vec::<u8>::with_capacity(1);
+    /// let ptr = vec.as_mut_ptr();
     /// unsafe { ptr.write(4) };
     /// ```
     #[clippy::version = "1.66.0"]