Codespell redfish-core spelling fixes

These spelling errors were found using
https://github.com/codespell-project/codespell
Tested: Top commit (along with this) was built and ran against
        validator.
Change-Id: Ic9dce27b1de8567eedf7753164ef564d3aedf8ca
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/redfish-core/lib/managers.hpp b/redfish-core/lib/managers.hpp
index 2ff65bf..2056d10 100644
--- a/redfish-core/lib/managers.hpp
+++ b/redfish-core/lib/managers.hpp
@@ -1497,7 +1497,7 @@
 
                 if (createNewObject && it.value() == nullptr)
                 {
-                    // can't delete a non-existant object
+                    // can't delete a non-existent object
                     messages::invalidObject(response->res, name);
                     continue;
                 }