Update documentation typo in `Inline Functions` in the `Leo Language Guide`

[a h]

Update documentation typo in Inline Functions in the Leo Language Guide

Task

Change:

The rules for functions (in the traditional sense) are as follows:

There are three variants of functions: transition, function, inline.
- transitions can only call functions and inlines.
- functions can only call inlines.
- inlines can only call inlines.
- Direct/indirect recursive calls are not allowed

To:

The rules for functions (in the traditional sense) are as follows:

There are three variants of functions: transition, helper, inline.
- transitions can only call functions and inlines
- helpers can only call inlines
- inlines can only call inlines
- direct/indirect recursive calls are not allowed