Corrected en-us growth rate names

[DDriggs00]

Previously, the names for the erratic and fluctuating en-us growth rates were listed incorrectly.

Note that Language id 6 (German) also needs corrected by someone who speaks German.

[magical]

Are Erratic/Fluctuating the official names now? If not, I'd prefer to stick with the current names, which are more descriptive.

It's fine that the prose name differs from the identifier.

[DDriggs00]

I can't confirm that it follows these naming conventions in game, simply because I've never seen growth rates directly named in game, but I am fairly certain that Fluctuating/Erratic are correct because Serebii, Bulbepedia, and PokemonDB all use this term, as well as when language_id 5 was added, it uses Fluctuating/Erratic.

[SnorlaxMonster]

I don't think there have ever been official names for experience groups. There might be some in the leaked source code, but I assume this repo would prefer not to rely on those leaks.

[DDriggs00]

Regardless of whether we go with erratic/fluctuating or "slow then very fast"/"fast then very slow", a change will be needed, as currently, both are used, depending on the language.