bpo-18610: Update wsgiref.validate docstring for wsgi.input read() (GH-11663)

diff --git a/Lib/wsgiref/validate.py b/Lib/wsgiref/validate.py
index 1c00bde..48ac007 100644
--- a/Lib/wsgiref/validate.py
+++ b/Lib/wsgiref/validate.py
@@ -77,7 +77,7 @@
 
 * That wsgi.input is used properly:
 
-  - .read() is called with zero or one argument
+  - .read() is called with exactly one argument
 
   - That it returns a string