Fix RST title underline length in file_utils.rst

The title underline was 61 characters but the title was 63 characters.
RST requires the underline to be at least as long as the title.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dpark01

docs/file_utils.rst

@@ -1,5 +1,5 @@
 file_utils.py - utilities to perform various file manipulations
-=============================================================
+================================================================
 
 .. argparse::
     :module:  viral_ngs.file_utils