chore: Update layers.rst (#3230)

* pin pyarrow in lambda layer

* Update layers.rst

Author: kukushking

building/lambda/build-lambda-layer.sh

@@ -11,7 +11,7 @@ popd
 rm -rf dist arrow
 
 export ARROW_HOME=$(pwd)/dist
-export ARROW_VERSION=21.0.0
+export ARROW_VERSION=20.0.0
 export LD_LIBRARY_PATH=$(pwd)/dist/lib:$LD_LIBRARY_PATH
 export CMAKE_PREFIX_PATH=$ARROW_HOME:$CMAKE_PREFIX_PATH
 export SETUPTOOLS_SCM_PRETEND_VERSION=$ARROW_VERSION