Fix details of sdist name format

Author: pfmoore

source/specifications/source-distribution-format.rst

@@ -24,8 +24,8 @@ Source distribution file name
 
 The file name of a sdist is not currently standardised, although the *de facto*
 form is ``{name}-{version}.tar.gz``, where ``{name}`` is the canonicalized form
-of the project name (see :pep:`503` for the canonicalization rules) and
-``{version}`` is the project version.
+of the project name (see :pep:`503` for the canonicalization rules) with ``-``
+characters replaced with ``_``, and ``{version}`` is the project version.
 
 The name and version components of the filename MUST match the values stored
 in the metadata contained in the file.