src/mte_tag: fix a typo for checktag instruction (#75)

Instruction mnemonics for checktag had `gentag` string due to
copy paste error. Fixing that.

Signed-off-by: Deepak Gupta <[email protected]>

Author: deepak0414

src/mte_tag.adoc

@@ -325,7 +325,7 @@ virtual address specified in `rs1`.
   {bits:  5, name: 'rs1', attr:['tagged_pointer']},
   {bits:  4, name: 'tag_imm4', attr:['chunk_count']},
   {bits:  1, name: '0', attr:['0']},
-  {bits:  7, name: '1000011', attr:['gentag']},
+  {bits:  7, name: '1000011', attr:['checktag']},
 ], config:{lanes: 1, hspace:1024}}
 ....