Update basics.md

I think, the correct form is `should be defined` OR `is defined`, but not `be defined`
Merge pull request #3162 from it3rmit/master

[ci skip] Update basics.md
Issue #3154 - DataColumn filter model validation error indication added
Issue #3154 - GridView - list of filter model validation errors added to summary
Improved RBAC docs
added more rbac doc [skip ci]
Fix typo
Merge pull request #3171 from gonimar/patch-15

Fix typo
typo fix [skip ci]
Fixes #2563: Theming is not working if the path map of the theme contains ".." or "." in the paths
Fixes #2592: added info on how to translate framework messages at application level
Fixed exception message to match the one in Component
InvalidCallException, InvalidParamException, UnknownMethodException are now extended from SPL BadMethodCallException
Added changelog for #3175 that fixes #2925
Merge pull request #3175 from yiisoft/fix-exception-hierarchy

Fix exception hierarchy
Gii missing space in crud update template [skip ci]
Merge pull request #3178 from lucianobaraglia/gii-ultraminor-fix

Gii missing space in crud update template [skip ci]
Issue #3154 - DataColumn checkForFilterErrors() now use Html::addCssClass
Render gridview table footer after table body
Fixed GridView cells calling order