org-edit-src-content-indentation

org-edit-src-content-indentation is a variable defined in `org-src.el'.
Its value is

2

Documentation:
Indentation for the content of a source code block.
This should be the number of spaces added to the indentation of the #+begin
line in order to compute the indentation of the block content after
editing it with M-x org-edit-src-code. Has no effect if
`org-src-preserve-indentation' is non-nil.

You can customize this variable.