Fix shared-link docs (#605)

Author: mwwoda

docs/usage/files.md

@@ -706,7 +706,7 @@ This method returns a `unicode` string containing the shared link URL.
 <!-- sample get_files_id get_shared_link -->
 ```python
 file_id = '11111'
-shared_link = client.file(file_id).shared_link
+shared_link = client.file(file_id).get().shared_link
 url = shared_link['download_url']
 ```
 

docs/usage/folders.md

@@ -241,7 +241,7 @@ This method returns a `unicode` string containing the shared link URL.
 <!-- sample get_folders_id get_shared_link -->
 ```python
 folder_id = '11111'
-shared_link = client.folder(folder_id).shared_link
+shared_link = client.folder(folder_id).get().shared_link
 url = shared_link['download_url']
 ```
 
@@ -505,4 +505,4 @@ with the ID of the folder lock.
 client.folder_lock(folder_lock_id='22222').delete()
 ```
 
-[delete_folder_lock]: https://box-python-sdk.readthedocs.io/en/latest/boxsdk.object.html#boxsdk.object.folder_lock.FolderLock
+[delete_folder_lock]: https://box-python-sdk.readthedocs.io/en/latest/boxsdk.object.html#boxsdk.object.folder_lock.FolderLock