Explicitely set text color to black

The akonotes plasmoid provided by KJots has a yellow background
(provided by an svg image) but takes the text color from the current
plasma theme.
This results in unreadable notes when you are using a (dark) theme with
a bright text color, e.g. Slim Glow or Produkt.

This patch fixes this by simply setting the text color to black.

BUG: 263816
FIXED-IN: 4.11.0
REVIEW: 111515