docs: Fix name of memory env var in README (#17)

The key is `AWS_LAMBDA_FUNCTION_MEMORY_SIZE`, not
`AWS_LAMBDA_MEMORY_SIZE` (see its usage in the code [1])

[1] https://github.com/aws/aws-lambda-runtime-interface-emulator/blob/6a74e68cb79a256b771210e7c46a0ca2e1cea61b/lambda/rapidcore/env/constants.go#L26

Co-authored-by: Jacob Fuss <32497805+jfuss@users.noreply.github.com>

Author: astahlman

README.md

@@ -124,7 +124,7 @@ You can configure timeout by setting AWS_LAMBDA_FUNCTION_TIMEOUT to the number o
 The rest of these Environment Variables can be set to match AWS Lambda's environment but are not required.
 * `AWS_LAMBDA_FUNCTION_VERSION`
 * `AWS_LAMBDA_FUNCTION_NAME`
-* `AWS_LAMBDA_MEMORY_SIZE`
+* `AWS_LAMBDA_FUNCTION_MEMORY_SIZE`
 
 ## Level of support