Fix typo in cpp readme (#4786)

Signed-off-by: Kyamran <[email protected]>

Author: nibblerenush

runtime/Cpp/cmake/README.md

@@ -137,7 +137,7 @@ By defualt the ANTLR C++ runtime source is cloned from GitHub. However, users ma
 
 Visual C++ compiler users may want to additionally define `ANTLR4_WITH_STATIC_CRT` before including the file. Set `ANTLR4_WITH_STATIC_CRT` to true if ANTLR4 C++ runtime library should be compiled with `/MT` flag, otherwise will be compiled with `/MD` flag. This variable has a default value of `OFF`. Changing `ANTLR4_WITH_STATIC_CRT` after building the library may require reinitialization of CMake or `clean` for the library to get rebuilt.
 
-You may need to modify your local copy of ExternalAntlr4Cpp.cpp to modify some build settings. For example, to specify the C++ standard to use when building the runtime, add `-DCMAKE_CXX_STANDARD:STRING=17` to `CMAKE_CACHE_ARGS`.
+You may need to modify your local copy of ExternalAntlr4Cpp.cmake to modify some build settings. For example, to specify the C++ standard to use when building the runtime, add `-DCMAKE_CXX_STANDARD:STRING=17` to `CMAKE_CACHE_ARGS`.
 
 ### Examples