fix charm build help message (#542)

`charm build --help`:

    Build charms will be placed into $CHARM_BUILD_DIR/{charm_name} (defaults to current directory)

This is not true, the real path in code is `/tmp/charm-builds/`

Signed-off-by: Joe Guo <guoqiao@gmail.com>

charmtools/build/builder.py

@@ -856,7 +856,7 @@ def main(args=None):
                                  "interface layers"),
         ("CHARM_BUILD_DIR", "Build charms will be placed into "
                             "$CHARM_BUILD_DIR/{charm_name} "
-                            "(defaults to current directory)"),
+                            "(defaults to /tmp/charm-builds/)"),
         ("JUJU_REPOSITORY", "Deprecated: If CHARM_BUILD_DIR is not set but "
                             "this is, built charms will be placed into "
                             "$JUJU_REPOSITORY/builds/{charm_name}"),