Address a gramar typo in the feature template

Author: rocky

.github/ISSUE_TEMPLATE/feature_request.md

@@ -6,7 +6,7 @@ labels: ''
 assignees: ''
 
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-*Note: If the feature is about adding or filling out existing deficiency in the Mathics3 language, please file this as an [issue](https://github.com/Mathics3/mathics-core/issues/new?assignees=&labels=&projects=&template=bug_report.md&title=).*
+*Note: If the feature is about adding or filling out an existing deficiency in the Mathics3 language, please file this as an [issue](https://github.com/Mathics3/mathics-core/issues/new?assignees=&labels=&projects=&template=bug_report.md&title=).*
 
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