Add missing ActionView helper number_to_human in guides

guides/source/action_view_helpers.md

@@ -333,6 +333,15 @@ Formats a number into a currency string (e.g., $13.65).
 number_to_currency(1234567890.50) # => $1,234,567,890.50
 ```
 
+#### number_to_human
+
+Pretty prints (formats and approximates) a number so it is more readable by users; useful for numbers that can get very large.
+
+```ruby
+number_to_human(1234)    # => 1.23 Thousand
+number_to_human(1234567) # => 1.23 Million
+```
+
 #### number_to_human_size
 
 Formats the bytes in size into a more understandable representation; useful for reporting file sizes to users.