DOC: intro: fix too long line.

dconv detected one line > 80 chars.
diff --git a/doc/intro.txt b/doc/intro.txt
index 8852757..19b4b1e 100644
--- a/doc/intro.txt
+++ b/doc/intro.txt
@@ -1355,7 +1355,7 @@
 
   - about 100 anti-virus proxies;
 
-  - and about 100 to 1000 application servers depending on the technology in use.
+  - and about 100-1000 application servers depending on the technology in use.
 
 
 3.6. How to get HAProxy