gh-40246: Make the completion of Sym be its formal series ring.
    
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```
sage: s = SymmetricFunctions(ZZ).s()
sage: s.completion() is s.formal_series_ring()
False
```
since we forgot to override the alias.

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preview.

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URL: #40246
Reported by: Travis Scrimshaw
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/combinat/sf/sfa.py

@@ -1672,10 +1672,14 @@ def formal_series_ring(self):
 
                 sage: type(L)
                 <class 'sage.rings.lazy_series_ring.LazySymmetricFunctions_with_category'>
+                sage: s.completion() is s.formal_series_ring()
+                True
             """
             from sage.rings.lazy_series_ring import LazySymmetricFunctions
             return LazySymmetricFunctions(self)
 
+        completion = formal_series_ring
+
 
 class FilteredSymmetricFunctionsBases(Category_realization_of_parent):
     r"""