release: Update package.xml version to 1.8.1

Author: nim65s

package.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0"?>
 <package format="3">
   <name>crocoddyl</name>
-  <version>1.8.0</version>
+  <version>1.8.1</version>
   <description>Crocoddyl optimal control library</description>
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