DOC: capitalize Gregorian and fix word order (#37697)
DOC: capitalize Gregorian and fix word order (#37697)

Gregorian is a proper noun
(https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar). Also,
"proleptic" is modifying the compound noun "Gregorian epoch" rather than
"Gregorian" modifying a "proleptic epoch".
dsm054/pandas,gfyoung/pandas,jreback/pandas,dsm054/pandas,jreback/pandas,dsm054/pandas,jorisvandenbossche/pandas,datapythonista/pandas,jreback/pandas,rs2/pandas,pandas-dev/pandas,jreback/pandas,pandas-dev/pandas,jorisvandenbossche/pandas,jorisvandenbossche/pandas,gfyoung/pandas,datapythonista/pandas,jorisvandenbossche/pandas,rs2/pandas,rs2/pandas,rs2/pandas,gfyoung/pandas,datapythonista/pandas,dsm054/pandas,dsm054/pandas,pandas-dev/pandas,jreback/pandas,pandas-dev/pandas,gfyoung/pandas,datapythonista/pandas,gfyoung/pandas