git svn: fix the final example in man page

git svn: fix the final example in man page

'git-remote add' creates a remote.origin.fetch entry in the config, we
want to replace this entry rather than add another one (which will
cause 'git fetch' to error).

This adds 'git config --remove-section remote.origin' after the fetch
for encouraging users to only use "git svn" for future updates.

[ew: rewording of commit message for present tense]

Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: StephenB <mail4stb@gmail.com>