Use https, not http for the learncpp url

Author: jimhester

vignettes/cpp11.Rmd

@@ -41,7 +41,7 @@ Typical bottlenecks that C++ can address include:
 The aim of this vignette is to discuss only those aspects of C++ and cpp11 that are absolutely necessary to help you eliminate bottlenecks in your code.
 We won't spend much time on advanced features like object-oriented programming or templates because the focus is on writing small, self-contained functions, not big programs.
 A working knowledge of C++ is helpful, but not essential.
-Many good tutorials and references are freely available, including <http://www.learncpp.com/> and <https://en.cppreference.com/w/cpp>.
+Many good tutorials and references are freely available, including <https://www.learncpp.com/> and <https://en.cppreference.com/w/cpp>.
 For more advanced topics, the _Effective C++_ series by Scott Meyers is a popular choice.
 
 ### Outline