verilog-indent-level-behavioral

verilog-indent-level-behavioral is a variable defined in `verilog-mode.el'.
Its value is

3

• This variable is safe as a file local variable if its value
  satisfies the predicate `integerp'.

Documentation:
Absolute indentation of first begin in a task or function block.
Set to 0 to get such code to start at the left side of the screen.

You can customize this variable.