docs(expression.py): correct grammar definition of lexer

Author: lovetheguitar

src/_pytest/mark/expression.py

@@ -5,7 +5,7 @@
 expression: expr? EOF
 expr:       and_expr ('or' and_expr)*
 and_expr:   not_expr ('and' not_expr)*
-not_expr:   'not' not_expr | '(' expr ')' | ident ( '(' name '=' value ( ', ' name '=' value )*  ')')*
+not_expr:   'not' not_expr | '(' expr ')' | ident ('(' name '=' value ( ', ' name '=' value )*  ')')?
 
 ident:      (\w|:|\+|-|\.|\[|\]|\\|/)+