specifications: clarify "sorted, set" to "sorted, collection"

This will hopefully clarify/eliminate a contradiction in terms
in the current specification text: the current text says
both "sorted" and "set", where "set" conventionally implies
an unordered collection.

In context that *appears* to have not been the intention,
since the context is the wheel filename encoding
and not the internal (i.e. parsed) representation
of a wheel filename's tags.

Signed-off-by: William Woodruff <william@yossarian.net>

Author: woodruffw

source/specifications/platform-compatibility-tags.rst

@@ -351,7 +351,7 @@ Compressed Tag Sets
 
 To allow for compact filenames of bdists that work with more than
 one compatibility tag triple, each tag in a filename can instead be a
-'.'-separated, sorted, set of tags.  For example, pip, a pure-Python
+'.'-separated, sorted, collection of tags.  For example, pip, a pure-Python
 package that is written to run under Python 2 and 3 with the same source
 code, could distribute a bdist with the tag ``py2.py3-none-any``.
 The full list of simple tags is::