Fix vim-plug specification to follow stable releases. Moving the same…

… tag is an antipattern that doesn't re-resolve with vim-plug, see this discussion for more detail (junegunn/vim-plug#720). Per vim-plug's maintainer's recommendation, use the 'tag' key instead with a shell wildcard. Wildcard should be '*.*.*' as that follows Black's versioning detailed here (https://black.readthedocs.io/en/latest/contributing/release_process.html\#cutting-a-release) and doesn't include current alpha releases.

docs/integrations/editors.md

@@ -125,7 +125,7 @@ Configuration:
 To install with [vim-plug](https://github.com/junegunn/vim-plug):
 
 ```
-Plug 'psf/black', { 'branch': 'stable' }
+Plug 'psf/black', { 'tag': '*.*.*' }
 ```
 
 ##### Vundle