Fix: Correct misspelling of OAuth in comments (#1762)

ServiceAccountCredentials.java repeatedly referred to "Oauth Endpoint" instead of "OAuth Endpoint" in comments. This commit corrects these misspellings.

Author: lsirac

oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java

@@ -582,7 +582,7 @@ public AccessToken refreshAccessToken() throws IOException {
  }
 
  /**
- * Returns a Google ID Token from either the Oauth or IAM Endpoint. For Credentials that are in
+ * Returns a Google ID Token from either the OAuth or IAM Endpoint. For Credentials that are in
  * the Google Default Universe (googleapis.com), the ID Token will be retrieved from the Oauth
  * Endpoint. Otherwise, it will be retrieved from the IAM Endpoint.
  *
@@ -601,7 +601,7 @@ public IdToken idTokenWithAudience(String targetAudience, List<Option> options)
  }
 
  /**
- * Uses the Oauth Endpoint to generate an ID token. Assertions and grant_type are sent in the
+ * Uses the OAuth Endpoint to generate an ID token. Assertions and grant_type are sent in the
  * request body.
  */
  private IdToken getIdTokenOauthEndpoint(String targetAudience) throws IOException {