ui: fix typo in docstring

Author: umarcor

vunit/ui/__init__.py

@@ -272,7 +272,7 @@ def add_source_files_from_csv(self, project_csv_path: Union[str, Path], vhdl_sta
                                  csv file
         :param vhdl_standard: The VHDL standard used to compile files,
                               if None, the VUNIT_VHDL_STANDARD environment variable is used
-        :returns: A list of files (:class `.SourceFileList`) that were added
+        :returns: A list of files (:class:`.SourceFileList`) that were added
 
         """
         libs: Set[str] = set()