Escape: Add two missing doublings of \ in some docstrings.

Escape: Add two missing doublings of \ in some docstrings.

The \\\\ in the source text inside """...""" turn into \\:

 >>> print("""
 ...  >>> DebianGitRepository.version_to_tag("%(version%-%\\\\%)s", "0-1.2.3")
 ... """)

  >>> DebianGitRepository.version_to_tag("%(version%-%\\%)s", "0-1.2.3")

 >>>

The \\ inside the quoted source text turn into \ when received by
version_to_tag.

Detected by flake8:
  ./gbp/deb/git.py:171:-37: W605 invalid escape sequence '\%'

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>