docs: add missing backslashes

Author: kenjis

user_guide_src/source/installation/upgrade_430.rst

@@ -15,10 +15,10 @@ Please refer to the upgrade instructions corresponding to your installation meth
 Trouble Shooting
 ****************
 
-Call to undefined method Composer\InstalledVersions::getAllRawData()
-====================================================================
+Call to undefined method Composer\\InstalledVersions::getAllRawData()
+=====================================================================
 
-Some users reported "*Fatal error: Uncaught Error: Call to undefined method Composer\InstalledVersions::getAllRawData()*" after upgrading with Composer.
+Some users reported "*Fatal error: Uncaught Error: Call to undefined method Composer\\InstalledVersions::getAllRawData()*" after upgrading with Composer.
 
 If you get the error, upgrade your ``composer`` tool, and delete the **vendor/**
 directory, and run ``composer update`` again.