Fix publishGuide task

[tylervz]

Fix the publishGuide task so that it succeeds for guides that are generated using Java 11, Grails 6.2.0 and Gradle 7.6.4.

Before this fix, I had to create a src/main/resources/img directory (in my Grails guide project) with an image file in it in order for ./gradlew publishGuide to work. Otherwise I'd get this error:

> Task :publishGuide FAILED

FAILURE: Build failed with an exception.

* What went wrong:
A problem was found with the configuration of task ':publishGuide' (type 'PublishGuide').
  - Type 'grails.doc.gradle.PublishGuide' property 'resourcesDir' specifies directory '/Users/tylervanzanten/src/grails_docs/adding-build-info/adding-build-info/build/resources/grails-guides-6.0.x/src/main/resources' which doesn't exist.
    
    Reason: An input file was expected to be present but it doesn't exist.
    
    Possible solutions:
      1. Make sure the directory exists before the task is called.
      2. Make sure that the task which produces the directory is declared as an input.
    
    Please refer to https://docs.gradle.org/7.6.4/userguide/validation_problems.html#input_file_does_not_exist for more details about this problem.

Creating a src/main/resources/img directory (in my Grails guide project) with an image file in it is a temporary workaround because then the copyLocalGuideImgResources task gets executed beforehand and creates the directory build/resources/grails-guides-6.0.x/src/main/resources.

[tylervz]

Upgrading grails-docs is not necessary for the fix to work. I just figured I might as well update it to the latest version.