cleanup: Fix documentation build error for ACQUIRE updates

cleanup: Fix documentation build error for ACQUIRE updates

Stephen reports:

Documentation/core-api/cleanup:7: include/linux/cleanup.h:73: ERROR: Unexpected indentation. [docutils]
Documentation/core-api/cleanup:7: include/linux/cleanup.h:74: WARNING: Block quote ends without a blank line; unexpected unindent. [docutils]

Which points out that the ACQUIRE() example in cleanup.h missed the "::"
suffix to mark the following text as a code-block.

Fixes: 857d18f23ab1 ("cleanup: Introduce ACQUIRE() and ACQUIRE_ERR() for conditional locks")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: http://lore.kernel.org/20250717173354.34375751@canb.auug.org.au
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20250717163036.1275791-1-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>