Fix negated factor condition example for not-a-and-not-b (#1683)

test_factor_ops makes it clear that `!a,!b` is "not-a OR not-b" and that
the "not-a AND not-b" (implied by the docs) should be `!a-!b`.

docs/config.rst

@@ -964,7 +964,7 @@ special case for a combination of factors. Here is how you do it:
         py{27,36}-sqlite: mock  # mocking sqlite in python 2.x & 3.6
         !py34-sqlite: mock      # mocking sqlite, except in python 3.4
         sqlite-!py34: mock      # (same as the line above)
-        !py34,!py36: enum34     # use if neither py34 nor py36 are in the env name
+        !py34-!py36: enum34     # use if neither py34 nor py36 are in the env name
 
 Take a look at the first ``deps`` line. It shows how you can special case
 something for a combination of factors, by just hyphenating the combining