Update base_classes/NXroot.nxdl.xml

Co-authored-by: Peter Chang <peter.chang@diamond.ac.uk>

Author: lukaspie

base_classes/NXroot.nxdl.xml

@@ -58,7 +58,7 @@
         <doc>
             A repository containing the application definitions
             used for creating this file.
-            If the NeXus_release attribute contains a commit distance and hash,
+            If the ``NeXus_release`` attribute contains a commit distance and hash,
             this should refer to this repository.
         </doc>
     </attribute>