Remove Array.toUnsafeBufferPointer

This commit removes the `Array.toUnsafeBufferPointer` usage from the
documentation.
This was a method that we used to have but removed in a previous commit.

Swift pointers have lifetimes that can be shorter than the lifetime of
the object that they are pointing to, so the returned pointer could
unpredictably become a dangling pointer.

The documentation has been updated with an alternative approach to
passing a Swift buffer pointer to Rust.

book/src/built-in/vec/README.md

@@ -52,12 +52,14 @@ func testMakeAVec () {
 
 func testMakeAnotherVec () {
     let initial: [Int16] = [3, 5, 7]
-    let vec: RustVec = get_vec_from_rust(initial.toUnsafeBufferPointer())
+    initial.withUnsafeBufferPointer({ initalPtr in
+        let vec: RustVec = get_vec_from_rust(initialPtr)
 
-    XCTAssertEqual(vec.len(), 3);
+        XCTAssertEqual(vec.len(), 3);
 
-	for (index, value) in vec.enumerate() {
-	    XCTAssertEqual(value, initial[index])
-	}
+	    for (index, value) in vec.enumerate() {
+	        XCTAssertEqual(value, initial[index])
+	    }
+    });
 }
 ```