Fix typo in envvar name.
diff --git a/Misc/SpecialBuilds.txt b/Misc/SpecialBuilds.txt
index 8111476..ae9ab4c 100644
--- a/Misc/SpecialBuilds.txt
+++ b/Misc/SpecialBuilds.txt
@@ -1,7 +1,7 @@
This file describes some special Python build types enabled via
compile-time preprocessor defines.
-It is best to define these options in the EXTRA_FLAGS environment variable;
+It is best to define these options in the EXTRA_CFLAGS environment variable;
``EXTRA_CFLAGS="-DPy_REF_DEBUG" ./configure``.
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