Oddly flags "Pi OS" to be changed to "PiOs"

**What got flagged?**

**Why is this incorrect?**

It turns out `PIO` is in the dictionary and has a plural, but that shouldn't override `Pi` followed by `OS`. The lettercase combos of the document words and the dictionary word should probably be taken into account.

Start by reproducing the flag on the phrase “Pi OS” and inspect the dictionary matching and case-comparison logic involved in recognizing “PIO.” Done means “Pi OS” is no longer flagged as “PiOs” while the existing plural handling for the dictionary word PIO remains correct.

Written by the indexing model from the issue text.