Fix regex in ecosystem identifiers

Co-authored-by: Malcolm Smith <smith@chaquo.com>

Author: jaimergp

peps/pep-0804.rst

@@ -295,7 +295,7 @@ The list of known ecosystems has two roles:
 Ecosystem identifiers
 ^^^^^^^^^^^^^^^^^^^^^
 
-The identifier MUST conform to this regex: ``[a-z0-9\-_.]+(\+[a-z0-9\-_.])?``.
+The identifier MUST conform to this regex: ``[a-z0-9\-_.]+(\+[a-z0-9\-_.]+)?``.
 In other words, a first field optionally followed by a second, separated
 by a ``+`` symbol.