Fix markdown-inline-code-face's :inherit attribute

[peniblec]

Hi! When inline code lost its foreground color some months ago,
I assumed it was a stylistic choice and did not question it; when
I tried to Customize the face recently, however, the Custom menu
spewed a broken face form.

AFAIK :inherit takes either a single face, or a sequence of faces, so
I guess the declaration ought to be fixed?

[jrblevin]

Apologies for the delay in incorporating this, but I wanted to go back and check the commit to see what I was thinking. Indeed, it was a typo. The idea was that all code faces (incl. inline code) would inherit from markdown-code-face. Thanks for catching this!