git-svn.txt: reword description of gc command
git-svn.txt: reword description of gc command

It's redundant to say that $GIT_DIR/svn/<refname>/unhandled.log or
$GIT_DIR/svn/<refname>/index is in .git/svn when $GIT_DIR is '.git', and
is wrong when $GIT_DIR is not '.git'

Also, a '/' was missing from the pathname $GIT_DIR/svn/<refname>/index .

Signed-off-by: Keshav Kini <72e3151be0cd6e86a4891765d5724258591bead8@gmail.com>
Signed-off-by: Eric Wong <8c2951295f8292b0c03727bc4652981172298eae@yhbt.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git