(CAT-2225) Update expected error message to account for changes to JSON

The error message being returned has changed, as the PDK is merely a wrapper for the JSON gem in this case and the error message is still understandable, we will simply update what is expected by the tests.

spec/unit/pdk/validate/metadata/metadata_syntax_validator_spec.rb

@@ -78,7 +78,7 @@
                                                      source: 'metadata-syntax',
                                                      state: :failure,
                                                      severity: 'error',
-                                                     message: a_string_matching(/unexpected token at/)
+                                                     message: a_string_matching(/expected \'\:\' after object key/)
                                                    })
         expect(return_value).to eq(1)
       end