input: Write a function that creates a web API using Flask to return the current date and time in the format "YYYY-MM-DD HH:MM:SS".
  assert isinstance(get_datetime(), str)
  assert len(get_datetime()) == 19
output: 
  from flask import Flask, jsonify
  from datetime import datetime
  
  app = Flask(__name__)
  
  @app.route('/get_datetime', methods=['GET'])
  def get_datetime():
      now = datetime.now()
      formatted_datetime = now.strftime("%Y-%m-%d %H:%M:%S")
      return jsonify(formatted_datetime)
  