Adjust readme to pylsp-mypy rename (#99)

Author: chaoflow

README.md

@@ -44,7 +44,7 @@ pip install -U setuptools
 Installing these plugins will add extra functionality to the language server:
 
 - [pyls-flake8](https://github.com/emanspeaks/pyls-flake8/): Error checking using [flake8](https://flake8.pycqa.org/en/latest/).
-- [mypy-ls](https://github.com/Richardk2n/mypy-ls): [MyPy](http://mypy-lang.org/) type checking for Python 3.
+- [pylsp-mypy](https://github.com/Richardk2n/pylsp-mypy): [MyPy](http://mypy-lang.org/) type checking for Python >=3.6.
 - [pyls-isort](https://github.com/paradoxxxzero/pyls-isort): code formatting using [isort](https://github.com/PyCQA/isort) (automatic import sorting).
 - [python-lsp-black](https://github.com/python-lsp/python-lsp-black): code formatting using [Black](https://github.com/psf/black).
 - [pyls-memestra](https://github.com/QuantStack/pyls-memestra): detecting the use of deprecated APIs.