Example (8) A 3-D histogram: Fix argument for "S"

[yvonnefroehlich]

Description of proposed changes

Following the documentation at https://docs.generic-mapping-tools.org/dev/plot3d.html#s:

-Sosize[c|i|p|q][+b|B[base]][+v|inz]

There should be a + before the b (or + B).

This PR updates the example "(8) A 3-D histogram".

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