Update sklmessages_si_LK.properties

[AquaCobalt]

I glossed over the existing translations and changed some words to simpler words. Some of the existing words are barely in use by Sinhala speakers in the modern day, so I changed them to words that most speakers would understand. Sinhala is still a difficult language to translate since its vocabulary is really limited compared to English. For example, for some words, just the writing script had to be changed, such as "Login" into "ලොගින්" (Latin to Sinhala script, but they both are pronounced the same). They were subtle changes and I've done these changes to make the translations more "natural", in the sense that they'd be words commonly used by most Sri Lankans instead of archaic words. That's pretty much it; but if this does get merged, it would provide for more easier-to-understand language.