Add reference to pep 776 for sys.platform value

Author: hoodmane

peps/pep-0783.rst

@@ -166,8 +166,11 @@ Package indexes SHOULD accept any wheel whose platform tag matches
 Dependency Specifier Markers
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-To check for the Emscripten platform in a dependency specifier, one can use
-``sys_platform == 'emscripten'`` (or its negation).
+According to `PEP 776
+<https://peps.python.org/pep-0776/#platform-identification>`__, in Emscripten
+Python `sys.platform` returns `"emscripten"`. To check for the Emscripten
+platform in a dependency specifier, one can use ``sys_platform == 'emscripten'``
+(or its negation).
 
 
 Trove Classifier