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.