RISC-V: Fix one nit indent issue of ustrunc pattern [NFC]

RISC-V: Fix one nit indent issue of ustrunc pattern [NFC]

Just notice the indent is not that right for ustrunc pattern from
the md files.  Thus, make it correct.  It is somehow very obvious
and will commit it after next 48H if no more comments.

gcc/ChangeLog:

	* config/riscv/autovec.md: Fix indent format issue.

Signed-off-by: Pan Li <pan2.li@intel.com>