Add "per capita"

**What got flagged?**
The text "per capita" was flagged. It's common in English: https://en.wikipedia.org/wiki/Per_capita.

**Why is this incorrect?**
It's correct as is.

**Example of correct usage:**
"The per capita CO2 emission has decreased".

Search the repository for the phrase "per capita" and the grammar-checking rule or word list that flags it. Confirm the existing behavior, update the relevant entry or rule so the phrase is accepted in the stated usage, and run the nearest available checker tests to verify the false positive is gone.

Written by the indexing model from the issue text.