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/index.rst b/doc/source/index.rst
index 784fc4d..3f903db 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -21,7 +21,7 @@
    zuul
    merger
    cloner
-   launchers
+   executors
    statsd
    client
    developer