Fix typo in box.schema.user.grant() (#2706)

Author: xuniq

doc/reference/reference_lua/box_schema/user_grant.rst

@@ -49,4 +49,4 @@ box.schema.user.grant()
         box.schema.user.grant('Lena', 'read,write', 'universe')
         box.schema.user.grant('Lena', 'Accountant')
         box.schema.user.grant('Lena', 'read,write,execute', 'universe')
-        box.schema.user.grant('X', 'read', 'universe', nil, {if_not_exists=true}))
+        box.schema.user.grant('X', 'read', 'universe', nil, {if_not_exists=true})