Update chapters/23.markdown
Changes from `:help call` to `:help :call`.
    
        | author | Martin Lundberg <martin.lundberg@gmail.com> | 
    
        | date | Wed, 23 Jan 2013 23:09:51 +0100 | 
    
    
        | parents | 0e78ce34b11d | 
    
        | children | 9f94e0c8d0f6 | 
    
        | branches/tags | (none) | 
    
        | files | chapters/23.markdown | 
Changes
    
--- a/chapters/23.markdown	Mon Nov 19 19:01:14 2012 -0500
+++ b/chapters/23.markdown	Wed Jan 23 23:09:51 2013 +0100
@@ -136,7 +136,7 @@
 Exercises
 ---------
 
-Read `:help call`.  Ignore anything about "ranges" for now.  How many arguments
+Read `:help :call`.  Ignore anything about "ranges" for now.  How many arguments
 can you pass to a function?  Is this surprising?
 
 Read the first paragraph of `:help E124` and find out what characters you're