Update reference.html - getTotalMarkers()

[thisisthefuture]

Description of getTotalMarkers() is incorrect.
The comment for getTotalMarkers() and in use shows the method returns the number of markers in the clusterer, not an array.

[googlebot]

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[thisisthefuture]

I signed it!

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CLAs look good, thanks!

[Remo]

@thisisthefuture unfortunately google doesn't support this repo anymore, but we've merged your changes to our own fork https://github.com/gmaps-marker-clusterer/gmaps-marker-clusterer
documentation can be found here https://gmaps-marker-clusterer.github.io/gmaps-marker-clusterer/#methods