Revise link_name restrictions wording

Author: traviscross

src/items/external-blocks.md

@@ -406,7 +406,7 @@ r[items.extern.attributes.link_name.syntax]
 The `link_name` attribute uses the [MetaNameValueStr] syntax.
 
 r[items.extern.attributes.link_name.invalid-names]
-The symbol name must not be the empty string, and must not contain any `U+0000` (NUL) bytes.
+The symbol name must not be the empty string or contain any `U+0000` (NUL) bytes.
 
 r[items.extern.attributes.link_name.allowed-positions]
 The `link_name` attribute may only be applied to a function or static item in an `extern` block.