Corrected parameter for downloadDocumentation (#83)

Author: jhaagmans

README.md

@@ -300,7 +300,7 @@ See the Serverless documentation for more information on [resource naming](https
 
 ### Download documentation from AWS API Gateway
 
-To download the deployed documentation you just need to use `serverless downloadDocumentation --outputFile=filename.ext`.
+To download the deployed documentation you just need to use `serverless downloadDocumentation --outputFileName=filename.ext`.
 For `yml` or `yaml` extensions application/yaml content will be downloaded from AWS. In any other case - application/json. 
 
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