docs: fix nodejs agent docker setup instructions

[kruskall]

See https://www.elastic.co/guide/en/apm/agent/nodejs/current/lambda.html

Current documentation appears as:

FROM docker.elastic.co/observability/apm-lambda-extension-x86_64:latest AS lambda-extension
AGENT_IMPORT
 
# FROM ...  <-- this is the base image of your Lambda function
 
COPY --from=lambda-extension /opt/elastic-apm-extension /opt/extensions/elastic-apm-extension
AGENT_COPY

[apmmachine]

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• Start Time: 2022-07-13T19:19:33.828+0000

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