Typo in Mathlib.Algebra.Polynomial.Degree.Definitions

Stream: mathlib4

[Ilmārs Cīrulis]

here

a polynomial is monic if its leading coefficient is 0 --- should be 1

[Ilmārs Cīrulis]

Apologies for not making PR

[Yaël Dillies]

Oops, indeed! Let's fix that quick

[Edward van de Meent]

classic off-by-one error :upside_down:

[Yaël Dillies]

(I am taking the opportunity to fully rewrite the module doc)