docs: Update doc name

We renamed 'docs/intro' to 'docs/index' a long time ago. Fix the
reference.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

README.rst

@@ -2,8 +2,7 @@
 git-pw
 ======
 
-.. NOTE: If editing this, be sure to update the line numbers in
-     'doc/introduction'
+.. NOTE: If editing this, be sure to update the line numbers in 'doc/index'
 
 .. image:: https://badge.fury.io/py/git-pw.svg
    :target: https://badge.fury.io/py/git-pw

docs/index.rst

@@ -5,7 +5,7 @@ Overview
 --------
 
 .. include:: ../README.rst
-   :start-line: 19
+   :start-line: 18
    :end-line: -7
 
 Usage