bump min c++ standard

docs.html

@@ -18597,7 +18597,7 @@ int main()
 If the above program is stored as <i>example.cpp</i>,
 under Linux and macOS it can be compiled using:
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-<ul><code>g++ example.cpp -o example -std=c++11 -O2 -larmadillo</code></ul>
+<ul><code>g++ example.cpp -o example -std=c++14 -O2 -larmadillo</code></ul>
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