fix(docs): point "create a histogram" link to proper id on Binning documentation

[mullim]

This fixes the "create a histogram" link in the Binning documentation (https://vega.github.io/vega-lite/docs/bin.html)

Before:

• https://vega.github.io/vega-lite/docs/bin.html#example

After:

• https://vega.github.io/vega-lite/docs/bin.html#histogram

[domoritz]

Thank you