Fix URL to demo of themes (#245)

Author: EQt

README.md

@@ -208,7 +208,7 @@ The drawback is of course that breakpoints in code that is stepped over are miss
 ### Syntax highlighting
 
 The source code preview is syntax highlighted and this highlighting has some options.
-The theme can be set by calling `Debugger.set_theme(theme)` where `theme` is a [Highlights.jl theme](https://juliadocs.github.io/Highlights.jl/stable/demo/themes.html).
+The theme can be set by calling `Debugger.set_theme(theme)` where `theme` is a [Highlights.jl theme](https://juliadocs.github.io/Highlights.jl/stable/demo/themes/).
 It can be completely turned off or alternatively, different quality settings for the colors might be chosen by calling `Debugger.set_highlight(opt)` where `opt` is a `Debugger.HighlightOption` enum.
 The choices are `HIGHLIGHT_OFF` `HIGHLIGHT_SYSTEM_COLORS`, `HIGHLIGHT_256_COLORS`, `HIGHLIGHT_24_BIT`. System colors works in pretty much all terminals, 256 in most terminals (with the exception of Windows)
 and 24 bit in some terminals.