Incorrect import path style in @lit-labs/observers documentation

[Masty88]

Current Behavior

The README example for MutationController shows an import path using snake_case:

import {MutationController} from '@lit-labs/observers/mutation_controller.js';

Expected Behavior

The import path should use kebab-case to match the actual file name convention:

import {MutationController} from '@lit-labs/observers/mutation-controller.js';

Impact

• Users copying the example from the documentation will get a 404 error since the file path doesn't exist
• Inconsistent with project's file naming conventions

Additional Context

This is a documentation-only fix to ensure examples in the documentation match the actual implementation.