docs(expression.py): fix typo

Author: lovetheguitar

src/_pytest/mark/expression.py

@@ -12,7 +12,7 @@
 The semantics are:
 
 - Empty expression evaluates to False.
-- ident evaluates to True of False according to a provided matcher function.
+- ident evaluates to True or False according to a provided matcher function.
 - or/and/not evaluate according to the usual boolean semantics.
 """