tests: fix docs compile syntax warning

tests: fix docs compile syntax warning

- make docs will generate syntax warning
/opt/vpp/docs/_scripts/siphon/generate.py:31: SyntaxWarning: invalid escape sequence '\s'
  siphon_block_start = re.compile("^\s*/\*\?\s*(.*)$")
/opt/vpp/docs/_scripts/siphon/generate.py:34: SyntaxWarning: invalid escape sequence '\s'
  siphon_block_stop = re.compile("^(.*)\s*\?\*/\s*$")
/opt/vpp/docs/_scripts/siphon/generate.py:42: SyntaxWarning: invalid escape sequence '\s'
  "(%s)\s*([a-zA-Z0-9_:]+)\s+(.*)\s*(%s)"
/opt/vpp/docs/_scripts/siphon/generate.py:47: SyntaxWarning: invalid escape sequence '\s'
  siphon_initializer = re.compile("\s*=")
/opt/vpp/docs/_scripts/siphon/generate_clicmd.py:23: SyntaxWarning: invalid escape sequence '\s'
  "(?P<m>VLIB_CLI_COMMAND)\s*" "[(](?P<name>[a-zA-Z0-9_]+)(,[^)]*)?[)]"
/opt/vpp/docs/_scripts/siphon/generate_syscfg.py:23: SyntaxWarning: invalid escape sequence '\s'
  "(?P<m>VLIB_CONFIG_FUNCTION)\s*"
/opt/vpp/docs/_scripts/siphon/generate_syscfg.py:24: SyntaxWarning: invalid escape sequence '\s'
  '[(](?P<fn>[a-zA-Z0-9_]+)\s*,\s*"(?P<name>[^"]*)"[)]'

Type: improvement

Change-Id: I6f7555cb0368a671a8bb164d51466d526895e925
Signed-off-by: fenglei <[email protected]>