[flang][NFC] Update document with current status

fir.dispatch codegen was done in D136189.

Author: clementval

flang/docs/PolymorphicEntities.md

@@ -861,7 +861,6 @@ Current list of TODOs in lowering:
 
 Current list of TODOs in code generation:
 
-- `flang/lib/Optimizer/CodeGen/CodeGen.cpp:897` not yet implemented: fir.dispatch codegen
 - `flang/lib/Optimizer/CodeGen/CodeGen.cpp:911` not yet implemented: fir.dispatch_table codegen
 - `flang/lib/Optimizer/CodeGen/CodeGen.cpp:924` not yet implemented: fir.dt_entry codegen
 - `flang/lib/Optimizer/CodeGen/CodeGen.cpp:2651` not yet implemented: fir.gentypedesc codegen