change 'additional' to 'extra'

Signed-off-by: Mehmet Enes <[email protected]>

Author: enesonus

content/master/cli/command-reference.md

@@ -829,7 +829,7 @@ Configuration/platform-ref-aws                             v0.9.0    True
 The `crossplane beta validate` command validates 
 [compositions]({{<ref "../concepts/compositions">}}) against provider or XRD 
 schemas using the Kubernetes API server's validation library
-with additional validation such as checking for unknown fields,
+with extra validation such as checking for unknown fields,
 a common source of difficult to debug issues in Crossplane.
 
 The `crossplane beta validate` command supports validating the following