doc: remove mention of renamed variable 'forcing_diag' (#657)

'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.

doc/source/user_guide/ug_troubleshooting.rst

@@ -126,7 +126,7 @@ conflicts in module dependencies.
 *print\_state* (**ice\_diagnostics.F90**)
     Print the ice state and forcing fields for a given grid cell.
 
-`forcing\_diag` = true (**ice\_in**)
+`debug\_forcing` = true (**ice\_in**)
     Print numerous diagnostic quantities associated with input forcing.
 
 `debug\_blocks` = true (**ice\_in**)