Update README to reflect new branch version (#947)

Author: lcgogo

README.md

@@ -40,7 +40,7 @@ Check the [releases] for the latest version.
 ``` console
 git clone \
   --single-branch \
-  --branch 8.3.3 \
+  --branch 8.4.0 \
   --recurse-submodules \
   https://github.com/cardano-foundation/cardano-graphql.git \
   && cd cardano-graphql