docs: grammar fix

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

content/client-server-api/modules/end_to_end_encryption.md

@@ -93,7 +93,7 @@ Example:
 ```
 
 `ed25519` and `curve25519` keys are used for [device keys](#device-keys).
-Additionally, `ed25519` keys are keys used for [cross-signing](#cross-signing).
+Additionally, `ed25519` keys are used for [cross-signing](#cross-signing).
 
 `signed_curve25519` keys are used for [one-time and fallback keys](#one-time-and-fallback-keys).