Add snippet tags. (dotnet#8042)

This fixes the current build warnings.

Author: BillWagner

snippets/fsharp/System/TupleT1,T2,T3,T4,T5,T6,T7,TRest/Item1/item1.fs

@@ -1,5 +1,6 @@
 open System
 
+// <Snippet1>
 let showPopulation year newPopulation =
  printfn $"""{year,5} {newPopulation,14:N0} {"n/a",10:P2}"""
 
@@ -64,4 +65,5 @@ do
 // 1970 1,511,462 -0.95 %
 // 1980 1,203,339 -2.04 %
 // 1990 1,027,974 -1.46 %
-// 2000 951,270 -0.75 %
+// 2000 951,270 -0.75 %
+// </Snippet1>