Fix: Error in documentation for FetchContent

[looopTools]

The current installation documentation for FetchContent with CMake, result in the following error Error:

CMake Error at /opt/homebrew/Cellar/cmake/3.28.1/share/cmake/Modules/FetchContent.cmake:1221 (message):
  No content details recorded for cli11
Call Stack (most recent call first):
  /opt/homebrew/Cellar/cmake/3.28.1/share/cmake/Modules/FetchContent.cmake:1740 (__FetchContent_getSavedDetails)
  /opt/homebrew/Cellar/cmake/3.28.1/share/cmake/Modules/FetchContent.cmake:2033 (FetchContent_Populate)
  CMakeLists.txt:17 (FetchContent_MakeAvailable)

The important part: No content details recorded for cli11.

I have changed the documentation such that the part which covers FetchContent works out of the box.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dc137f0) 100.00% compared to head (aba8211) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #969   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines         4506      4506           
=========================================
  Hits          4506      4506           

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