Update a4 and a9 in the English version

[zbraiterman]

I've proposed some updates in A4 and A9 of the English version. The updates include the following changes to the titles themselves:

• Change lack-of-resources-and-rate-limiting to lack-of-resource-and-rate-limiting (changing the plural noun, "resources", to the singular noun, "resource").
• Change improper-assets-management to improper-asset-management (changing the plural noun, "assets", to the singular noun, "asset").

[PauloASilva]

Hi @zbraiterman,
Thanks for your interest.

I believe your PR is not OK:

• Instead of creating new markdown files and copy&paste the content, you should have used git mv to rename the original ones.
• There are a few references to these sections throughout the document which were not updated accordingly.
• Consider applying suggested changes to all available translations.
• PR should target the develop branch instead of master per de CONTRIBUTING.md.

Please consider updating your PR accordingly if you really believe the 2019 edition benefits from suggested changes.
As it happened before (1, 2) such changes should be subject of community review and approval.

Cheers,
Paulo A. Silva