doc: remove mention of renamed variable 'forcing_diag'

[phil-blain]

'forcing_diag' was renamed to 'debug_forcing' in d6eb125 (Add new unit
tests sumchk and bcstchk and update tests (#606), 2021-06-09), but this
instance was not renamed in the doc.

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'forcing_diag' was renamed to 'debug_forcing' in d6eb125 (Add new unit
tests sumchk and bcstchk and update tests (#606), 2021-06-09), but this
instance was not renamed in the doc.

[apcraig]

Thanks for catching this @phil-blain, we should get rid of the "\_" in general. I don't think it's required anymore. Having the \ makes grepping for things in the documentation more difficult. Will merge once checks are complete.