fix: correct typos in README and variables.tf descriptions

[dadangnh]

This pull request focuses on correcting typos in documentation and variable descriptions across multiple modules (mysql, postgresql, private_service_access, and safer_mysql). The changes improve clarity and consistency by fixing spelling errors and ensuring accurate descriptions.

Documentation Typo Fixes:

MySQL Module:

• Fixed spelling of "settings" in the backup_configuration description in modules/mysql/README.md.
• Corrected "default" spelling in the tier description in modules/mysql/README.md and modules/mysql/variables.tf. [1] [2]

PostgreSQL Module:

• Fixed spelling of "settings" in the backup_configuration description in modules/postgresql/README.md and modules/postgresql/variables.tf. [1] [2]
• Corrected "default" spelling in the tier description in modules/postgresql/README.md and modules/postgresql/variables.tf. [1] [2]

Private Service Access Module:

• Fixed spelling of "project" in the project_id description in modules/private_service_access/README.md and modules/private_service_access/variables.tf. [1] [2]

Safer MySQL Module:

• Fixed spelling of "settings" in the backup_configuration description in modules/safer_mysql/README.md and modules/safer_mysql/variables.tf. [1] [2]

[dadangnh]

Hi @imrannayer @ayushmjain @q2w , kindly review this PR