Correct typo: "they are" -> "there are" (#725)

Author: jdufresne

docs/topics/migration-from-PHPExcel.md

@@ -3,7 +3,7 @@
 PhpSpreadsheet introduced many breaking changes by introducing
 namespaces and renaming some classes. To help you migrate existing
 project, a tool was written to replace all references to PHPExcel
-classes to their new names. But they are also manual changes that
+classes to their new names. But there are also manual changes that
 need to be done.
 
 ## Automated tool