Two crows being an attempted murder

The joke is a play on words [Cambridge Dictionary] on various definitions of murder.

A group of crows is called a murder. [Wikipedia]

Two is not quite a group, hence an attempted murder.

To further beat the joke to death, murder also means homicide, and attempted murder is a crime in British and United States' penal codes. The unusual combination of birds and crime adds to the humor as a non-sequitur.

It’s not really a language joke—it’s a cultural joke, I think.

There is a superstition that three crows seen together bodes murder. Therefore, two crows seen together is one crow short of murder—or “attempted murder”.

It’s true that a group of crows is sometimes called a “murder”, but this seems to stem from the superstition, and not the other way around. Given the precise number two in the joke, I would guess that the superstition is being referred to, not the word.