input: Write a python function to detect occurrences of the word "coronavirus" in text files within a specified directory.
  assert detect_coronavirus('test_dir_1') == 15
  assert detect_coronavirus('test_dir_2') == 0
  assert detect_coronavirus('test_dir_3') == 42
output: def detect_coronavirus(directory):
      import os
      
      count = 0
      for filename in os.listdir(directory):
          if filename.endswith(".txt"):
              with open(os.path.join(directory, filename), 'r') as file:
                  content = file.read().lower()
                  count += content.count("coronavirus")
      
      return count