Tutorial - clearly specify that function names should start with a lower case letter

Fixes #3199

Author: samaaron

etc/doc/tutorial/05.5-Functions.md

@@ -24,8 +24,9 @@ we wish to give to our function. We didn't have to call it `foo`, we could
 have called it anything we want such as `bar`, `baz` or ideally
 something meaningful to you like `main_section` or `lead_riff`.
 
-Remember to prepend a colon `:` to the name of your function when you define
-it.
+Remember to prepend a colon `:` to the name of your function when you
+define it. Function names should also start with a lower case letter, so
+use `:foo` rather than `:Foo`.
 
 ## Calling functions