[LyX/2.1.x] Fix grammar error spotted by Sian Mountbatten

[Juergen Spitzmueller]

commit d85bc320e724b57f73e12fdfe8f003ac59044e74
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Sat Oct 25 13:50:37 2014 +0200

    Fix grammar error spotted by Sian Mountbatten

diff --git a/lib/doc/Intro.lyx b/lib/doc/Intro.lyx
index 93fb60f..1a66dc4 100644
--- a/lib/doc/Intro.lyx
+++ b/lib/doc/Intro.lyx
@@ -870,7 +870,7 @@ Navigate
 \begin_layout Standard
 In the printed manuals, all cross-references appear as the actual numbers
  for a chapter, section, subsection, and so on.
- In the LyX-file, however, all cross-references appear as a light-grey boxes
+ In the LyX-file, however, all cross-references appear as light-grey boxes
  like the following: 
 \begin_inset Graphics
        filename clipart/referenceQt4.png