Replaced the term "inheritable" with "inherited by default" on the Learn CSS page on inheritance

[jernwerber]

The Learn CSS article on inheritance (005) uses the term "inheritable" which is both potentially misleading and not used in the CSS spec. This is potentially misleading because the word "inheritable" can be interpreted as "able to be inherited" as opposed to how it is being used here, which is "inherited by default". Additionally the CSS spec does not use the word "inheritable" instead referring to properties that inherit their values by default as "inherited properties".

Fixes #10109

Changes proposed in this pull request:

• Everywhere the word "inheritable" is used, I have replaced it with "inherited by default", which is how the word seems to have been used on this page. Though the CSS spec refers to "inherited properties", I believe that using the term "inherited by default" is better for learners because it is more explicit as to what is occurring.

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