Update db-migrations.md

Minor syntax changes.
Update composer-asset-plugin version to beta4

Have not tested that beta4 works, honestly just trying out the GitHub edit button :P
Update db-migrations.md
Merge pull request #6188 from cdvrooman/patch-26

[skip ci] Update db-migrations.md
Merge pull request #6189 from rawtaz/patch-1

[ci skip] Update composer-asset-plugin version to beta4 in start-installation.md
Fixes #6189
docs/guide-ja/runtime-logging.md - prepared for translation [ci skip]
docs/guide-ja/runtime-logging.md - WIP [ci skip]
docs/guide-ja/runtime-logging.md - WIP [ci skip]
docs/guide-ja/runtime-logging.md - completed [ci skip]
Merge pull request #6194 from softark/docs-guide-ja-2014-11-24

docs/guide-ja/runtime-logging.md added [ci skip]
Merge pull request #7 from yiisoft/master

Update 201411241000