FIX greedy re_yaml_schema pattern error

using `\s{4}` for indent also captures leading `\n`s, trimming too much newlines.

- changed to: `[ ]{4}`
- tests
	1. `PageOperationMixin.re_yaml_schema` does not capture leading newlines
	2. `PageOperationMixin.render_body` does not trim too much, preserving separate paragraphs
Merge pull request #79 from jangxyz/FIX-yaml-schema-tab-and-space-2

FIX greedy re_yaml_schema pattern error