Update install docs for pip as well

Author: Cito

README.md

@@ -46,7 +46,7 @@ README and the corresponding tests in parallel.
 
 GraphQL-core 3 can be installed from PyPI using the built-in pip command:
 
-    python -m pip install "graphql-core>=3"
+    python -m pip install "graphql-core>=3a"
 
 Alternatively, you can also use [pipenv](https://docs.pipenv.org/) for installation in a
 virtual environment:

docs/intro.rst

@@ -31,7 +31,7 @@ Getting started
 
 You can install GraphQL-core 3 using pip_::
 
-    pip install "graphql-core>=3"
+    pip install "graphql-core>=3a"
 
 You can also install GraphQL-core 3 with pipenv_, if you prefer that::