Do not use the <tt> tag in documentation

[christophertubbs]

The <tt> tag shows up in some of the documentation, but it is not a supported tag in HTML5, as a result, we have some documentation rendered that does not render the <tt> tag correctly and literally reads <tt>production</tt> instead of production.

This might be fixed with a simple find and replace with <tt>-><code> and </tt>-></code> or just use something like the <samp> tag if you're feeling fancy.

An example of the issue may be found at: https://noaa-owp.github.io/ngen/md_doc_2_g_i_t___u_s_a_g_e.html#autotoc_md173