Fix spelling

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: matthewhilton

.github/instructions/vendor.instructions.md

@@ -1,5 +1,5 @@
 When bumping simplesaml as a dependency we need to review the following
 Make sure we have committed the full vendor folder, so it can be installed as expected
 Make sure that the version of simplesamlphp installed is the one listed in thirdpartylibs.xml
-Make sure all the patches in the patches folder have sucessfully applied
+Make sure all the patches in the patches folder have successfully applied
 Make sure there has been a version bump in the main version.php file