Doc improvements on user guide
This change:
* fixes some spelling issues
* externally links to a document describing what a 'git ref' is
* links to executor document at the first reference to it
* links to independent pipeline at the first reference to it
Change-Id: I3f2247deac15fd15c9dead6cba537d7b17e8491f
diff --git a/doc/source/user/index.rst b/doc/source/user/index.rst
index 3eca04b..8c7308b 100644
--- a/doc/source/user/index.rst
+++ b/doc/source/user/index.rst
@@ -4,7 +4,7 @@
This guide is for all users of Zuul. If you work on a project where
Zuul is used to drive automation (whether that's testing proposed
changes, building artifacts, or deploying builds), this guide will
-help you understand the concepts that underly Zuul, and how to
+help you understand the concepts that underlie Zuul, and how to
configure it to meet your needs.