Need to ensure rendered as pH in html

[andypbarrett]

I have the term in all lowercase - ph. This needs to be pH. In modern chemical notation, the lowercase p is shorthand for the negative logarithm in base 10.

I would also suggest that the "power of hydrogen" is deleted as it does not add much to the definition. Instead the definition starts with "a measure of the acidity or basicity... of aqueous solutions - this needs an explanation"

https://github.com/nsidc/nsidc_cryosphere_glossary/blob/5c1d18e0d6e8d59837e84d5904fe50bd3bc2b9d5/glossary/p/ph.yml#L1C1-L4C42