Add missing "not" in "input.setFiles" algorithm check when "files" parameter is greater than 1. (#673)

Author: lutien

index.bs

@@ -10513,7 +10513,7 @@ The [=remote end steps=] given |session| and |command parameters| are:
    return [=error=] with [=error code=] [=unable to set file input=].
 
 1. If the [=set/size=] of |files| is greater than 1 and |element|'s
-   <{input/multiple}> attribute is set, return [=error=] with [=error code=]
+   <{input/multiple}> attribute is not set, return [=error=] with [=error code=]
    [=unable to set file input=].
 
 1. Let |files| be the value of the |command parameters|["<code>files</code>"]