fix DOI reference

Author: mariojmdavid

content/00.front-matter.md

@@ -8,7 +8,7 @@
 ![](images/logo-SYNERGY.png){height="50px"}    
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-_A DOI-citable version of this manuscript is available at <http://hdl.handle.net/10261/214441>_.
+_A DOI-citable version of this manuscript is available at <https://doi.org/10.20350/digitalCSIC/12533>_.
 
 {## Template to insert build date and source ##}
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