Fix bind_parameter_index/2 return type spec

Callers assume it always returns a non_neg_integer. The | {:error, reason()}
union was incorrect and misleading.

Author: mjc

lib/exqlite/sqlite3_nif.ex

@@ -87,8 +87,7 @@ defmodule Exqlite.Sqlite3NIF do
   @spec bind_parameter_count(statement) :: non_neg_integer() | {:error, reason()}
   def bind_parameter_count(_stmt), do: :erlang.nif_error(:not_loaded)
 
-  @spec bind_parameter_index(statement, String.t()) ::
-          non_neg_integer() | {:error, reason()}
+  @spec bind_parameter_index(statement, String.t()) :: non_neg_integer()
   def bind_parameter_index(_stmt, _name), do: :erlang.nif_error(:not_loaded)
 
   @spec bind_text(statement, non_neg_integer, String.t()) :: integer()