fix(grammar): a to an in Authentication-Concepts (#7977)

Co-authored-by: Edison Zhang <edisonzz@amazon.com>

Author: edisooon

src/pages/[platform]/build-a-backend/auth/concepts/index.mdx

@@ -35,7 +35,7 @@ export function getStaticProps() {
 
 Amplify helps you secure your application while providing an easy sign-in experience for your users. This experience is influenced by your security strategy. This security strategy includes the authentication method, security credentials, and enabling additional verification when needed.
 
-- _Authentication_ is a process to validate **who you are** (abbreviated as _AuthN_). The system that does this validation is referred to as an Identity Provider or IdP. This can be your own self-hosted IdP or a cloud service. Oftentimes, this IdP is a external provider such as Apple, Facebook, Google, or Amazon.
+- _Authentication_ is a process to validate **who you are** (abbreviated as _AuthN_). The system that does this validation is referred to as an Identity Provider or IdP. This can be your own self-hosted IdP or a cloud service. Oftentimes, this IdP is an external provider such as Apple, Facebook, Google, or Amazon.
 - _Authorization_ is the process of validating **what you can access** (abbreviated as _AuthZ_). This is sometimes done by looking at tokens with custom logic, predefined rules, or signed requests with policies.
 
 Common authentication methods and associated risks include: