doc: Fix typo in rcu_head comments

 *

 * This guarantee is important for few reasons:

 *  - future call_rcu_lazy() will make use of lower bits in the pointer;

 *  - the structure shares storage spacer in struct page with @compound_head,

 *  - the structure shares storage space in struct page with @compound_head,

 *    which encode PageTail() in bit 0. The guarantee is needed to avoid

 *    false-positive PageTail().

 */