Remove --valid flag from xmllint

The existing call to xmllint returns an error status, the error message is obscured by the output, but it writes to stderr:

    explicit-failures-markup.xml:3: validity error : Validation failed: no DTD found !
     xsi:noNamespaceSchemaLocation="explicit-failures.xsd">

According to the xmllint man page the `--valid` flag is used for validating using a DTD, and we don't have a DTD. The `--schema` flag is sufficient for validation, so I think the solution is just to remove the `--valid` flag.

status/explicit-failures-markup.xml

@@ -7,7 +7,7 @@
 
     Locally, the xmlint tool can be used:
 
-        xmllint <two-dashes>valid explicit-failures-markup.xml <two-dashes>schema explicit-failures.xsd
+        xmllint explicit-failures-markup.xml <two-dashes>schema explicit-failures.xsd
 
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