under-the-hood.md: correction about commit objects

[mmesiti]

According to, e.g., https://git-scm.com/book/en/v2/Git-Internals-Git-Objects, "blobs" are one type of git object, alongside with 'tree objects' and 'commit objects' (and 'tag objects').

I think that calling a commit a 'blob' is a misuse of the term 'blob'. In the git man pages these terms are used with a specific meaning, misnaming things could lead to misunderstanding.

(Awesome material though, I will use it)