docs: Changed externalUser() to externalUser in External Authentication documentation (#9982)

Changed externalUser() to externalUser in external-authentication.md
file.

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# Which Problems Are Solved

Replace this example text with a concise list of problems that this PR
solves.
For example:
- External Authentication documentation states that to retrieve the
external user in the post authentication flow, use externalUser().
However, it needs to be externalUser (the method).

# How the Problems Are Solved

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introduces.
For example:
- Changing externalUser() to externalUser in the
external-authentication.md file solves this issue.

# Additional Changes

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but are related.
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- No additional changes were made.

# Additional Context

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- Closes #9893

docs/docs/apis/actions/external-authentication.md

@@ -27,7 +27,7 @@ The first parameter contains the following fields
   - `idToken` *string*  
     The id token provided by the identity provider.
   - `v1`
-    - `externalUser()` [*externalUser*](./objects#external-user)
+    - `externalUser` [*externalUser*](./objects#external-user)
     - `authError` *string*  
       This is a verification errors string representation. If the verification succeeds, this is "none"
     - `authRequest` [*auth request*](/docs/apis/actions/objects#auth-request)