input: Write a python function to filter out employees whose salary is below the mean salary.
  assert filter_below_mean_salary(pd.DataFrame({'salary': [3000, 4000, 5000, 6000]})).equals(pd.DataFrame({'salary': [5000, 6000]}))
  assert filter_below_mean_salary(pd.DataFrame({'salary': [1500, 2500, 3500, 4500, 5500]})).equals(pd.DataFrame({'salary': [4500, 5500]}))
  assert filter_below_mean_salary(pd.DataFrame({'salary': [1000, 2000, 3000, 4000, 5000, 6000]})).equals(pd.DataFrame({'salary': [4000, 5000, 6000]}))
output: import pandas as pd
  
  def filter_below_mean_salary(df):
      mean_salary = df['salary'].mean()
      return df[df['salary'] >= mean_salary]