Fix a test failure using Ruby 3.3.0dev

This PR fixes the following test failure using Ruby 3.3.0dev:

```console
$ ruby -v
ruby 3.3.0dev (2023-08-14T15:48:39Z master 52837fcec2) [x86_64-darwin22]
$ bundle exec rspec spec/faraday_spec.rb

Randomized with seed 57031

Faraday
  has a version number
  proxies to default_connection
    uses method_missing on Faraday if there is no proxyable method (FAILED - 1)
    proxies methods that exist on the default_connection
    proxied methods can be accessed

Failures:

  1) Faraday proxies to default_connection uses method_missing on Faraday if there is no proxyable method
     Failure/Error:
       expect { Faraday.this_method_does_not_exist }.to raise_error(
         NoMethodError, expected_message
       )

       expected NoMethodError with "undefined method `this_method_does_not_exist' for Faraday:Module",
       got #<NoMethodError: undefined method `this_method_does_not_exist' for module Faraday> with backtrace:
         # ./lib/faraday.rb:147:in `method_missing'
         # ./spec/faraday_spec.rb:27:in `block (4 levels) in <top (required)>'
         # ./spec/faraday_spec.rb:27:in `block (3 levels) in <top (required)>'
     # ./spec/faraday_spec.rb:27:in `block (3 levels) in <top (required)>'
```

That error message has been changed by ruby/ruby@e7b8d32e in Ruby 3.3.0dev.

cf. https://bugs.ruby-lang.org/issues/18285

So the test error message is changed:

Ruby 3.2 or lower:

```
undefined method `this_method_does_not_exist' for Faraday:Module
```

Ruby 3.3.0dev:

```
NoMethodError: undefined method `this_method_does_not_exist' for module Faraday
```

spec/faraday_spec.rb

@@ -18,10 +18,13 @@
     end
 
     it 'uses method_missing on Faraday if there is no proxyable method' do
-      expect { Faraday.this_method_does_not_exist }.to raise_error(
-        NoMethodError,
-        "undefined method `this_method_does_not_exist' for Faraday:Module"
-      )
+      expected_message = if RUBY_VERSION >= '3.3'
+                           "undefined method `this_method_does_not_exist' for module Faraday"
+                         else
+                           "undefined method `this_method_does_not_exist' for Faraday:Module"
+                         end
+
+      expect { Faraday.this_method_does_not_exist }.to raise_error(NoMethodError, expected_message)
     end
 
     it 'proxied methods can be accessed' do