Fixes #6976
[BUG][NODEJS] nodejs-express-server API docs path mapping inconsistencies

Updated /api-doc to /api-docs to be consistent with README.md instructions with other generators.

PR checklist

• Read the contribution guidelines.
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• Run the shell script ./bin/generate-samples.sh to update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/config/java*. For Windows users, please run the script in Git BASH.
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@CodeNinjai (2017/07) @frol (2017/07) @cliffano (2017/07)