Explicitly set encoding when opening files in system_doxygen.py (Robo…

…tLocomotion#14536)

doc/system_doxygen.py

@@ -134,7 +134,7 @@ def process_doxygen_system_tags(s):
         raise RuntimeError(f"Wrong number of arguments.\n{__doc__}")
 
     # Load the file in batch into a string.
-    with open(sys.argv[1], 'r') as f:
+    with open(sys.argv[1], 'r', encoding='utf-8') as f:
         s = f.read()
 
     s = process_doxygen_system_tags(s)