Fix cmake warning due to incorrect case in find_package

Fixes this warning message:
```
CMake Warning (dev) at /usr/local/Cellar/cmake/3.19.6/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:426 (message):
  The package name passed to `find_package_handle_standard_args` (Gtest) does
  not match the name of the calling package (GTest).  This can lead to
  problems in calling code that expects `find_package` result variables
  (e.g., `_FOUND`) to follow a certain pattern.
```

Author: glennguy

CMakeLists.txt

@@ -103,8 +103,8 @@ include(CPack)
 if(NOT CMAKE_CROSSCOMPILING)
   list(APPEND CMAKE_CTEST_ARGUMENTS "--output-on-failure")
   enable_testing()
-  include(FindGTest)
-  find_package(GTest REQUIRED)
+  include(FindGtest)
+  find_package(Gtest REQUIRED)
   include_directories(${GTEST_INCLUDE_DIRS})
   add_subdirectory(src/test)
 endif()