doc(data/polynomial/basic): fix docstring (#15814)

src/data/polynomial/basic.lean

@@ -32,7 +32,7 @@ the polynomials. For instance,
 
 Polynomials are defined using `add_monoid_algebra R ℕ`, where `R` is a semiring.
 The variable `X` commutes with every polynomial `p`: lemma `X_mul` proves the identity
-`X * p = `p * X`.  The relationship to `add_monoid_algebra R ℕ` is through a structure
+`X * p = p * X`.  The relationship to `add_monoid_algebra R ℕ` is through a structure
 to make polynomials irreducible from the point of view of the kernel. Most operations
 are irreducible since Lean can not compute anyway with `add_monoid_algebra`. There are two
 exceptions that we make semireducible: