Fixing acceptance tests for RF 3.1 (#1255)

Author: aaltat

atest/acceptance/keywords/elements.robot

@@ -121,5 +121,5 @@ Cover Element can cover just one element
     Should Be equal As Integers  ${count}  1
 
 Cover Elements should throw exception when locator is invalid
-    Run Keyword And Expect Error  No element with locator '//img[@src="inexistent"]' found.
+    Run Keyword And Expect Error  No element with locator '//img?@src="inexistent"?' found.
     ...  Cover Element  //img[@src="inexistent"]