build(submodules): fix ffmpeg branch in gitmodules

[ReenigneArcher]

Description

This is a minor correction to the .gitmodules file, which was missed in #136.

Screenshot

Issues Fixed or Closed

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch
must be updated before it can be merged. You must also
Allow edits from maintainers.

• I want maintainers to keep my branch updated

[cgutman]

This fix is fine, but maybe we don't want Dependabot spamming us every day with changes from FFmpeg master. What does it do if we specify no branch there?

[ReenigneArcher]

No branch will be the same as the default branch.

We definitely don't have to address each PR... we could also disable dependabot just for ffmpeg if we wanted to.