Fix typo of branch name "main"

What does this MR do and why?

There are two main in this paragraph (as shown in the screenshot), and they mean different things.

• The first main: represents the database, similar to ci
• The second main: indicates branch name

The second main needs to be changed to master because GitLab's default branch is master. We can also see in the code that the default value of TARGET here is master.

# https://gitlab.com/gitlab-org/gitlab/-/blob/master/scripts/migration_schema_validator.rb#L75

def target_branch
  ENV['CI_MERGE_REQUEST_TARGET_BRANCH_NAME'] || ENV['TARGET'] || ENV['CI_DEFAULT_BRANCH'] || 'master'
end

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cc @mtan-gitlab