doc: Correct reference to cluster attribute auto_scaling_disk_gb_enabled

[SimonPerryman]

Description

Currently in the basic cluster documentation there is a warning about ignoring changes in the terraform lifecycle when disk_gb_enabled is enabled. However, that is the attribute name for the advanced cluster, not the basic cluster. I've fixed the reference in the warning message to reference the attribute name of the basic cluster attribute..

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

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• I have signed the MongoDB CLA
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• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
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Further comments

[andreaangiolillo]

Thanks for your contribution! 💯