patch 8.1.1937: errors when using javascriptreact

Problem:    Errors when using javascriptreact.
Solution:   Use ":runtime" instead of ":source". (closes #4875)
diff --git a/runtime/ftplugin/javascriptreact.vim b/runtime/ftplugin/javascriptreact.vim
index ea5c9f5..af318a0 100644
--- a/runtime/ftplugin/javascriptreact.vim
+++ b/runtime/ftplugin/javascriptreact.vim
@@ -1,2 +1,2 @@
 " Placeholder for backwards compatilibity: .jsx used to stand for JavaScript.
-source %:h/javascript.vim
+runtime! ftplugin/javascript.vim