fix erroneous code path within show_sym (JuliaLang#38830)

fix erroneous code path within `show_sym`

currently, `show_sym` can recur into `show(::IO, String)`, but it's a
bit "erroneous" since `is_syntactic_operator(::String)` is not defined.

base/show.jl

@@ -1445,12 +1445,12 @@ end
 # Print `sym` as it would appear as an identifier name in code
 # * Print valid identifiers & operators literally; also macros names if allow_macroname=true
 # * Escape invalid identifiers with var"" syntax
-function show_sym(io::IO, sym; allow_macroname=false)
+function show_sym(io::IO, sym::Symbol; allow_macroname=false)
     if is_valid_identifier(sym)
         print(io, sym)
     elseif allow_macroname && (sym_str = string(sym); startswith(sym_str, '@'))
         print(io, '@')
-        show_sym(io, sym_str[2:end])
+        show_sym(io, Symbol(sym_str[2:end]))
     else
         print(io, "var", repr(string(sym)))
     end