docs: fix minor typo (#3311)

docs/compiling.rst

@@ -347,7 +347,7 @@ standard explicitly with
 
 
     set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ version selection")  # or 11, 14, 17, 20
     set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ version selection")  # or 11, 14, 17, 20
     set(CMAKE_CXX_STANDARD_REQUIRED ON)  # optional, ensure standard is supported
     set(CMAKE_CXX_STANDARD_REQUIRED ON)  # optional, ensure standard is supported
-    set(CMAKE_CXX_EXTENSIONS OFF)  # optional, keep compiler extensionsn off
+    set(CMAKE_CXX_EXTENSIONS OFF)  # optional, keep compiler extensions off
 
 
 The variables can also be set when calling CMake from the command line using
 The variables can also be set when calling CMake from the command line using
 the ``-D<variable>=<value>`` flag. You can also manually set ``CXX_STANDARD``
 the ``-D<variable>=<value>`` flag. You can also manually set ``CXX_STANDARD``