Rename zuul-launcher to zuul-executor

To avoid confusion with nodepool-launcher, we've decided to rename
zuul-launcher to zuul-executor.

Change-Id: I7d03cf0f0093400f4ba2e4beb1c92694224a3e8c
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/doc/source/merger.rst b/doc/source/merger.rst
index 82e204b..d8de702 100644
--- a/doc/source/merger.rst
+++ b/doc/source/merger.rst
@@ -30,7 +30,7 @@
 Git reference of the form ``refs/zuul/<branch>/Z<random sha1>``.
 Preparing the workspace is then a simple matter of fetching that ref
 and checking it out.  The parameters that provide this information are
-described in :ref:`launchers`.
+described in :ref:`executors`.
 
 These references need to be made available via a Git repository that
 is available to workers (such as Jenkins).  This is accomplished by