Say that distribution names should be lowercase in wheel filenames

Author: takluyver

source/specifications/binary-distribution-format.rst

@@ -159,8 +159,9 @@ As the components of the filename are separated by a dash (``-``, HYPHEN-MINUS),
 this character cannot appear within any component. This is handled as follows:
 
 - In distribution names, any run of ``-_.`` characters (HYPHEN-MINUS, LOW LINE
-  and FULL STOP) should be replaced with ``_`` (LOW LINE). This is equivalent
-  to :pep:`503` normalisation followed by replacing ``-`` with ``_``.
+  and FULL STOP) should be replaced with ``_`` (LOW LINE), and uppercase
+  characters should be replaced with corresponding lowercase ones. This is
+  equivalent to :pep:`503` normalisation followed by replacing ``-`` with ``_``.
 - Version numbers should be normalised according to :pep:`440`. Normalised
   version numbers cannot contain ``-``.
 - The remaining components may not contain ``-`` characters, so no escaping