Update agent_step! function in Boltzmann-with-grid example

[def-mycroft]

While making the edits for pull request 26, I noticed that the agent_step! function needs to be modified in the latter part of the tutorial.

In the file boltzmann_example01.md, the first part of the example deals with a Boltzmann model that does not have a spatial grid. Later in the example, a spatial structure is added and the model is modified so that, instead of giving wealth to a random agent, agents give their wealth to an agent that is "nearby".

The agent_step! function at the end of boltzmann_example01.md is the same function as was used earlier in the example. It needs to be modified to accurately reflect the description in the example, then the corresponding script in examples/ should be updated as well (note that after pull request 26 is merged, there will be two scripts in examples/, boltzmann_wealth_distribution.jl and boltzmann_wealth_distribution_with_grid.jl).

[def-mycroft]

kavir1698 shared code in PR #26, I'm going to fix and close this issue soon.

[def-mycroft]

kavir1698 shared agent_step! code with a couple of lines that move the agent after every step, might need to add those back in before closing, see PR #26