Update valid colors link in THEMES.md to ratatui (#2511)

Author: acuteenvy

THEMES.md

@@ -29,7 +29,7 @@ Notes:
 
 * rgb colors might not be supported in every terminal.
 * using a color like `yellow` might appear in whatever your terminal/theme defines for `yellow`
-* valid colors can be found in tui-rs' [Color](https://docs.rs/tui/0.12.0/tui/style/enum.Color.html) struct.
+* valid colors can be found in ratatui's [Color](https://docs.rs/ratatui/latest/ratatui/style/enum.Color.html) struct.
 * all customizable theme elements can be found in [`style.rs` in the `impl Default for Theme` block](https://github.com/extrawurst/gitui/blob/master/src/ui/style.rs#L305)
 
 ## Customizing line breaks