configure.ac: Clean up help string for --with-statdpath

Neither m4 nor the vim colorizer like unbalanced single quotes.
Similar change as commit 34ee5730.

Also, the default value of the command line option is conventionally
listed in the right-hand side argument of AC_HELP_STRING.

Introduced by commit 5c3e2665: "statd: Decouple statd's state
directory from the NFS state directory," (September 20, 2011).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>