Incorrect explanation for error E0117

[cr1901]

As of commit 1e5450d, the detailed explanation for error E0117 begins with:

The `Drop` trait was implemented on a non-struct type.

Erroneous code example:

impl Drop for u32 {}

The "The Drop trait was implemented on a non-struct type" sentence appears to be meant for E0120, but accidentally got included as part of the E0117 text.

I'm not sure how upstream wants to proceed here, but note that the example impl Drop for u32 {} will trigger both E0117 and E0120. So keeping this example- while changing the text- is probably fine.

I lost more time than I'd like to admit to misreading the docs here :P.