Fix typo in 778.removal.md
`hikari.impl.bot.GatewayBotApp` > `hikari.impl.bot.GatewayBot`
Removal of deprecated `hikari.impl.bot.BotApp` and `hikari.traits.BotAware`
-- Use `hikari.impl.bot.GatewayBotApp` and `hikari.traits.GatewayBotAware` instead respectively
+- Use `hikari.impl.bot.GatewayBot` and `hikari.traits.GatewayBotAware` instead respectively
