Fix yaml files codestyle

.hadolint.yaml

+---
 ignored:
     - DL3006
     - DL3008

.readthedocs.yml

+---
 # .readthedocs.yml
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details

.travis.yml

+---
 dist: bionic
 language: python
 python:
@@ -43,4 +44,5 @@ stages:
 
 env:
   global:
+    # yamllint disable-line
     secure: 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

examples/docker-compose/notebook/letsencrypt-notebook.yml

+---
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 

examples/docker-compose/notebook/notebook.yml

+---
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 

examples/docker-compose/notebook/secure-notebook.yml

+---
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.