Update reference.html

[duncancumming]

Fixing return type and description for getTotalMarkers method

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[duncancumming]

I signed it!

[googlebot]

CLAs look good, thanks!

[Remo]

@duncancumming this repo isn't maintained anymore. Check this for the currently most active fork: https://github.com/gmaps-marker-clusterer/gmaps-marker-clusterer
We've also created a documentation where this issue is already fixed: https://gmaps-marker-clusterer.github.io/gmaps-marker-clusterer/#reference
If you have any further pull requests, send them there, I'd be happy to review them.