tests/resource/aws_codebuild_project: Fix typo in error message testing for buildspec
tests/resource/aws_codebuild_project: Fix typo in error message testing for buildspec

Reference: https://github.com/terraform-providers/terraform-provider-aws/pull/12590

Output from acceptance testing:

```
--- PASS: TestAccAWSCodeBuildProject_Source_Type_NoSourceInvalid (12.19s)
```

terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws