Fix "slashaed" typo (#2205)

# Description

This merely fixes a typo in the documentation, replacing the typo
"slashaed" with "slashed". Since external entities use the comments for
explanations of events, this will then be shown externally. I noticed
this when reviewing [this
event](https://polkadot.subscan.io/extrinsic/0xb6bc1e3abde0c2ed9c500c74cfc64cdb8179e5d9af97f4bf53242ce4cdd15a1d?event=18064194-6)
on Subscan.

This is not related to any other issues or PRs.