Fix invalid regex in scripts/python/md-split.py (#2303)

The `[` character doesn't need to be escaped.

The `]` character needs to be escaped as `\\]` or as `\]` in a raw string.

scripts/python/md-split.py

@@ -181,7 +181,7 @@ def is_inside_code(line, indent_depth):
 def stripped(line):
     # Remove well-formed html tags, fixing mistakes by legitimate users
     sline = TAG_REGEX.sub('', line)
-    sline = re.sub('[()\[\]#*]', ' ', line)
+    sline = re.sub(r'[()[\]#*]', ' ', line)
     return sline
 
 def dedent(line, indent_depth):