fix: update image color for dark mode in the "shared module" section.

[Tony133]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Docs
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

• Added "illustrative-image" class css for dark mode in the illustrative-image "shared module" so that it is in line with other images when there is dark mode ( see screenshot )

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

lgtm