Typo in usbd_core.c

[Devilbinder]

In the function description of USBD_DeInit. There is a missing e.

Please see this commit https://github.com/STMicroelectronics/STM32CubeF1/commit/a188ada2a5aab5358cb0ce1e6e5ab77d7d0d014e

I will PR it if it is not on the internal tracker.

[Devilbinder]

PR created #53. Assuming adding this to To do is a green lite.