doc(number_theory/legendre_symbol/jacobi_symbol): remove now spurious…

… reference to 'subscript J' (#16541)

This PR just fixes the docstring for the notation. The reference to "no subscript J in unicode" doesn't make sense anymore after the notation was changed to `J(a | b)`.

src/number_theory/legendre_symbol/jacobi_symbol.lean

@@ -67,7 +67,7 @@ Jacobi symbol when `b` is odd and gives less meaningful values when it is not (e
 is `1` when `b = 0`). This is called `jacobi_sym a b`.
 
 We define localized notation (locale `number_theory_symbols`) `J(a | b)` for the Jacobi
-symbol `jacobi_sym a b`. (Unfortunately, there is no subscript "J" in unicode.)
+symbol `jacobi_sym a b`.
 -/
 
 open nat zmod