x86/sgx: Fix a typo in the kernel-doc comment for enum sgx_attribute

x86/sgx: Fix a typo in the kernel-doc comment for enum sgx_attribute

Use the exact enum name when documenting "enum sgx_attribute" to fix a
warning if the file is fed into kernel-doc processing:

  WARNING: ./arch/x86/include/asm/sgx.h:139 expecting prototype for enum
           sgx_attributes. Prototype was for enum sgx_attribute instead

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251112160708.1343355-6-seanjc%40google.com