Help: Fix elseif/endif typo

In CMake 3.13.x, we had this evaluation for if() conditions:

    if(a) elseif(b) else(a) endif(a)

The sensible intention of the change in commit c2efb3ef (Help: Revise
docs on Scripting Commands, 2018-10-16, v3.14.0-rc1~505^2) was:

> "endif", "endfunction" etc: Explain that the argument is optional and
> maintained for compatibility only

Instead of "endif", it ended up being written to the documentation as
"elseif" by the commit author (oops) to if()'s page.

    if(a) elseif(a!?) else() endif(a?)

Truthfully, endif()'s parameter should be an optional verbatim repeat
and not elseif()'s. If it wasn't, elseif() would be described to be the
same as if(). The rightful intended description is:

    if(a) elseif(b) else() endif()

Fix that typo.
Status Job ID Name Coverage
  External
passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager