I usually edit the formatting of the posts so I mostly add "Fixed formatting".
Can there be a way to set this as the default edit summary so that, whenever I am editing a post, this text will be filled in the edit summary field so I do not have to type it each time I edit?
In fact, this feature already exists (kind of).
When you edit a post and write nothing into the edit summary field, the system will automatically choose an appropriate edit summary.
I have "tested" this with your post:
When you look at my revision, you'll see this "default" edit summary:
added 26 characters in body
However, if you want a more descriptive default summary, you need to choose an user script. For example the one rene proposed in the comments: AutoReviewComments - Pro-forma comments for SE.
Also, this only works, if you have "full editing rights" over the post. This means that it is either (a) your own post, (b) you have global editing privilege (commonly 2k reputation) or (c) the post is community wiki and you have global CW editing privileges (commonly at 100 reputation).
