readme: fix instruction to enable topic
We moved the source code in 85390446f8c1 but we messed up the readme update. We
are fixing it here.
--- a/README.md Mon Mar 21 21:30:14 2016 -0700
+++ b/README.md Tue Mar 22 15:29:05 2016 -0700
@@ -12,4 +12,4 @@
local directory, and add that directory to your `.hgrc`:
[extensions]
- topics=path/to/hg-topics/src
+ topics=path/to/hg-topics/hgext3rd/topic/