Update Doxygen version

Author: h2zero

docs/Doxyfile

@@ -38,7 +38,7 @@ PROJECT_NAME           = "esp-nimble-cpp / NimBLE-Arduino"
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 1.3.0
+PROJECT_NUMBER         = 1.3.1
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a