Fix units for SPS30 number concentration sensors

[kahrendt]

What does this implement/fix?

This PR corrects the units for the number concentration sensors to match the datasheet, page 2. They measure in #/cm³ instead of the current #/m³. I modified the incorrect unit in const.py, as this was the only component using it.

Types of changes

• Bugfix (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)
• Other

Related issue or feature (if applicable): fixes esphome/issues#4873

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#3220

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040

Example entry for config.yaml:

# Example config.yaml

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[probot-esphome]

Hey there @martgras, mind taking a look at this pull request as it has been labeled with an integration (sps30) you are listed as a code owner for? Thanks!
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