minor #12719 [minor] remove usage of Kernel::$rootDir from docs (jzawadzki)

This PR was submitted for the 5.0 branch but it was squashed and merged into the 4.3 branch instead (closes #12719).

Discussion
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[minor] remove usage of Kernel::$rootDir from docs

As per symfony/symfony#28810 $rootDir has been removed in 5.0 - use getProjectDir in docs instead - small change to be consistent

Commits
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9da96e1 [minor] remove usage of Kernel::$rootDir from docs

Author: javiereguiluz

reference/configuration/kernel.rst

@@ -105,7 +105,7 @@ method to return the right project directory::
 Cache Directory
 ~~~~~~~~~~~~~~~
 
-**type**: ``string`` **default**: ``$this->rootDir/cache/$this->environment``
+**type**: ``string`` **default**: ``$this->getProjectDir()/var/cache/$this->environment``
 
 This returns the absolute path of the cache directory of your Symfony project.
 It's calculated automatically based on the current
@@ -119,7 +119,7 @@ cache directory.
 Log Directory
 ~~~~~~~~~~~~~
 
-**type**: ``string`` **default**: ``$this->rootDir/log``
+**type**: ``string`` **default**: ``$this->getProjectDir()/var/log``
 
 .. deprecated:: 4.2