docs: fix installation instruction (#372)

Install instructions in the README.md fails with an error: „Could not find a version that satisfies the requirement graphene-sqlalchemy>=3“ This is because v3 is in beta. Therefore, installing with '--pre' fixes the problem.
graphql-python · Jan 2, 2023 · a03e74d · a03e74d
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commit a03e74d
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diff --git a/README.md b/README.md
@@ -18,7 +18,7 @@ A [SQLAlchemy](http://www.sqlalchemy.org/) integration for [Graphene](http://gra
 For installing Graphene, just run this command in your shell.
 
 ```bash
-pip install "graphene-sqlalchemy>=3"
+pip install --pre "graphene-sqlalchemy"
 ```
 
 ## Examples