[docs] In the CMake primer, correct the description of the ARGV/ARGN variables.
[docs] In the CMake primer, correct the description of the ARGV/ARGN variables.

ARGN is the sublist of unnamed arguments, not the count of the arguments.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@311632 91177308-0d34-0410-b5e6-96231b3b80d8

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