Update README about the header parsing policy for markdown ToC

[ryul1206]

The current code doesn't care about a neighborhood level but only cares about the level difference.
It means ### may be possible to appear after #.

# single_gen.py
.... 
92:    def append_toc(self, head, level):
93:        # if (self.prev_level + 1) == level:  # new child
94:        if self.prev_level < level:  # new child
95:            pass
...

However, the README only describes the strict rules below:

...
(Note) If you skip the title level of the markdown marked with #, an error will occur. In other words, the subtitle of ## must be ###.
...

Soft examples like below are much more flexible, so we need to test this.

# Title
### Subtitle1
### Subtitle2

[ryul1206]

On second thought, this example below doesn't make sense because it will crash an indentation of ToC.

# Title
### Subtitle1
### Subtitle2

So I revert the code from self.prev_level < level to (self.prev_level + 1) == level and close this issue.