doc: fix hypercall API doxygen errors

Documentation for parameters must match exactly in spelling and case.
Parameter named "vcpu" was incorrectly documented as "VCPU", and
parameter named "param" was documented as "param's".

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

Author: dbkinder

hypervisor/include/common/hypercall.h

@@ -329,7 +329,7 @@ int64_t hcall_setup_sbuf(struct vm *vm, uint64_t param);
 /**
  * @brief Switch VCPU state between Normal/Secure World.
  *
- * @param VCPU Pointer to VCPU data structure
+ * @param vcpu Pointer to VCPU data structure
  *
  * @return 0 on success, non-zero on error.
  */
@@ -338,8 +338,8 @@ int64_t hcall_world_switch(struct vcpu *vcpu);
 /**
  * @brief Initialize environment for Trusty-OS on a VCPU.
  *
- * @param VCPU Pointer to VCPU data structure
- * @param param's guest physical address. This gpa points to
+ * @param vcpu Pointer to VCPU data structure
+ * @param param guest physical address. This gpa points to
  *              struct trusty_boot_param
  *
  * @return 0 on success, non-zero on error.