dirstate: clarify a `hg update` invocation in a test

It is common for readers of that test to confuse the `hg co` call with a `hg
commit`, while it actually means `hg checkout`, an alias for the more common
`hg update.

So let us use the clearer version.

Differential Revision: https://phab.mercurial-scm.org/D11777

tests/test-dirstate-race.t

@@ -18,7 +18,7 b''
 Do we ever miss a sub-second change?:
 
   $ for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20; do
-  >     hg co -qC 0
+  >     hg update -qC 0
   >     echo b > a
   >     hg st
   > done