fix nonsetable_type_hint_handler (#55962)

The current implementation is wrong, causing it to display inappropriate
hints like the following:
```julia
julia> s = Some("foo");

julia> s[] = "bar"
ERROR: MethodError: no method matching setindex!(::Some{String}, ::String)
The function `setindex!` exists, but no method is defined for this combination of argument types.
You attempted to index the type String, rather than an instance of the type. Make sure you create the type using its constructor: d = String([...]) rather than d = String
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1
```

base/errorshow.jl

@@ -1052,7 +1052,7 @@ function nonsetable_type_hint_handler(io, ex, arg_types, kwargs)
             print(io, "\nAre you trying to index into an array? For multi-dimensional arrays, separate the indices with commas: ")
             printstyled(io, "a[1, 2]", color=:cyan)
             print(io, " rather than a[1][2]")
-        else isType(T)
+        elseif isType(T)
             Tx = T.parameters[1]
             print(io, "\nYou attempted to index the type $Tx, rather than an instance of the type. Make sure you create the type using its constructor: ")
             printstyled(io, "d = $Tx([...])", color=:cyan)

test/errorshow.jl

@@ -739,31 +739,33 @@ end
 pop!(Base.Experimental._hint_handlers[DomainError])  # order is undefined, don't copy this
 
 struct ANumber <: Number end
-let err_str
-    err_str = @except_str ANumber()(3 + 4) MethodError
+let err_str = @except_str ANumber()(3 + 4) MethodError
     @test occursin("objects of type $(curmod_prefix)ANumber are not callable", err_str)
     @test count(==("Maybe you forgot to use an operator such as *, ^, %, / etc. ?"), split(err_str, '\n')) == 1
     # issue 40478
     err_str = @except_str ANumber()(3 + 4) MethodError
     @test count(==("Maybe you forgot to use an operator such as *, ^, %, / etc. ?"), split(err_str, '\n')) == 1
 end
 
-let err_str
-    a = [1 2; 3 4];
+let a = [1 2; 3 4];
     err_str = @except_str (a[1][2] = 5) MethodError
     @test occursin("\nAre you trying to index into an array? For multi-dimensional arrays, separate the indices with commas: ", err_str)
     @test occursin("a[1, 2]", err_str)
     @test occursin("rather than a[1][2]", err_str)
 end
 
-let err_str
-    d = Dict
+let d = Dict
     err_str = @except_str (d[1] = 5) MethodError
     @test occursin("\nYou attempted to index the type Dict, rather than an instance of the type. Make sure you create the type using its constructor: ", err_str)
     @test occursin("d = Dict([...])", err_str)
     @test occursin(" rather than d = Dict", err_str)
 end
 
+let s = Some("foo")
+    err_str = @except_str (s[] = "bar") MethodError
+    @test !occursin("You attempted to index the type String", err_str)
+end
+
 # Execute backtrace once before checking formatting, see #38858
 backtrace()