Update CHANGELOG.md

Author: jalsti

CHANGELOG.md

@@ -20,6 +20,7 @@ Unreleased (aka. GitHub master)
 
 - In *Collapsible Headings*, get the notebook version from `sys_info` if possible, resolving an incompatibility with *notebook* versions 6.5.x [#1617](https://github.com/ipython-contrib/jupyter_contrib_nbextensions/pull/1617).
 - Add short execution threshold for ExecutionTime plugin. Excution time will not show on cells that executed faster than the given threshold [#1336](https://github.com/ipython-contrib/jupyter_contrib_nbextensions/pull/1336).
+- Fix bug on nbconvert: No module named 'notebook.services' when running `jupyter nbconvert --execute --allow-errors --to pdf myfile.ipynb` [#1663](https://github.com/ipython-contrib/jupyter_contrib_nbextensions/pull/1663)
 
 0.7.0
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