[BUG] Fix supported syntax

[dani-petrov]

Hello, I will attach a screenshot of the currently supported syntax for Alter Table -> modify column:

Looking at the circled part, the correct syntax for a MODIFY [COLUMN] statement is together with [IF EXISTS] and not [IF NOT EXISTS]. This leads to failed parsing. Is it possible to allow both IF NOT EXISTS and IF EXISTS?
Thank you!

[manticore-projects]

Greetings! Your observation is certainly correct and I will have to take a look at that.