Fix monkeypatch doc

`delenv` is incorrectly documented.

src/_pytest/monkeypatch.py

@@ -22,7 +22,7 @@ def monkeypatch():
         monkeypatch.setitem(mapping, name, value)
         monkeypatch.delitem(obj, name, raising=True)
         monkeypatch.setenv(name, value, prepend=False)
-        monkeypatch.delenv(name, value, raising=True)
+        monkeypatch.delenv(name, raising=True)
         monkeypatch.syspath_prepend(path)
         monkeypatch.chdir(path)