guide.md: adding missing title

Pull Request: #346

Author: NPalopoli

_extras/guide.md

@@ -533,6 +533,8 @@ yearly_data_arg_test(surveys_df, end_year=2001)
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 {: .language-python}
 
+### Functions Modifications Challenges
+
 * Rewrite the `one_year_csv_writer` and `yearly_data_csv_writer` functions to have keyword arguments with default values.
 
 ~~~