[optional.optional.ref.general] Remove redundant access-specifier (cplusplus#9019)

Author: Andreas-Krug

source/utilities.tex

@@ -4795,7 +4795,6 @@
     using iterator   = @\impdefnc@;  // present only if \tcode{T} is an object type other than an array
                                                 // of unknown bound; see~\ref{optional.ref.iterators}
 
-  public:
     // \ref{optional.ref.ctor}, constructors
     constexpr optional() noexcept = default;
     constexpr optional(nullopt_t) noexcept : optional() {}