fix problem with string_concatenation_hint_handler (#49016)

Quiets an error on CI when running missing tests.

Refs #45823

(cherry picked from commit 3919a89)

base/errorshow.jl

@@ -925,7 +925,7 @@ Experimental.register_error_hint(noncallable_number_hint_handler, MethodError)
 # (probably attempting concatenation)
 function string_concatenation_hint_handler(io, ex, arg_types, kwargs)
     @nospecialize
-    if (ex.f == +) && all(i -> i <: AbstractString, arg_types)
+    if (ex.f === +) && all(i -> i <: AbstractString, arg_types)
         print(io, "\nString concatenation is performed with ")
         printstyled(io, "*", color=:cyan)
         print(io, " (See also: https://docs.julialang.org/en/v1/manual/strings/#man-concatenation).")