Remove broken ref in introduction, and fix wording in prior sentence. (#637)

The "other tools" reference doesn't exist, so the link is removed. The wording in the prior sentence about the existence of other packages that provide alternative frontends for Shiny is improved in wording.

Author: bryce-carson

introduction.Rmd

@@ -75,8 +75,7 @@ There are also a number of important topics specific to Shiny that I don't cover
 
 -   This book only covers the built-in user interface toolkit.
     This doesn't provide the sexiest possible design, but it's simple to learn and gets you a long way.
-    If you have additional needs (or just get bored with the defaults), there are number of other packages that provide alternative front ends.
-    See Section \@ref(other-tools) for more details.
+    If you have additional needs (or just get bored with the defaults), there are other packages that provide alternative front ends.
 
 -   Deployment of Shiny apps.
     Putting Shiny "into production" is outside the scope of this book because it hugely varies from company to company, and much of it is unrelated to R (the majority of challenges tend to be cultural or organisational, not technical).