Fix DeprecationWarning about 'invalid escape sequence' in tools/docs Python scripts (#2281)

diff --git a/tools/mkdoc.py b/tools/mkdoc.py
index c85e09d..cd982e0 100755
--- a/tools/mkdoc.py
+++ b/tools/mkdoc.py
@@ -106,8 +106,8 @@
         result = result2
 
     # Doxygen tags
-    cpp_group = '([\w:]+)'
-    param_group = '([\[\w:\]]+)'
+    cpp_group = r'([\w:]+)'
+    param_group = r'([\[\w:\]]+)'
 
     s = result
     s = re.sub(r'\\c\s+%s' % cpp_group, r'``\1``', s)