Fix minor d -> A typo in continuous batching blog

[chauvinSimon]

@remi-or @ArthurZucker @McPatate thanks for the great content and format!

One correction I am not 100% about:

• Just below the first figure, it says: "Computing QKT requires O(n²d) operations".
• Because Q and K are of shape n*A, shouldn't it say "requires O(n²A) operations"?
• I was confused when reading the first time, but maybe d is used because the h heads are considered and d = Ah.

[pcuenca]

In the context of the post, I think the line is referring to a single attention head. Personally, I think we could just use O(n^2) without loss of generality :)