fix 'varibale' typo (#596)

Author: SylvainJuge

docs/monitoring-aws-lambda.asciidoc

@@ -67,7 +67,7 @@ The configuration options for the APM agents are documented in the corresponding
 
 NOTE: Some APM agent configuration options don't make sense when the APM agent is running in a Lambda environment.
 For example, instead of using the Python APM agent configuration variable, `verify_server_cert`, you must use the
-`ELASTIC_APM_LAMBDA_VERIFY_SERVER_CERT` varibale described below.
+`ELASTIC_APM_LAMBDA_VERIFY_SERVER_CERT` variable described below.
 
 NOTE: APM Central configuration is not supported when using the Elastic APM AWS Lambda extension