doc: fix documentation error in usage section

The example provided in the documentation used a different name for g_pFuncTableInfo, whic caused compilation to fail in GPAIL_LoadApi due to an undefined symbol:

    /tmp/ccuhxdcO.o: In function `GPAIL_LoadApi(GPA_API_Type, char const*)':
    main.cpp:(.text+0x502): undefined reference to `g_pFuncTableInfo'

The documentation now includes the proper snippet taken from `include/gpu_perf_api_interface_loader.h`

Author: Roberto Santalla

docs/sphinx/source/usage.rst

@@ -65,9 +65,10 @@ initialize the DirectX 12 version of GPA:
 
     #include "GPAInterfaceLoader.h"
 
+    #ifdef __cplusplus
     GPAApiManager* GPAApiManager::m_pGpaApiManager = nullptr;
-
-    GPAFunctionTable* pGpaFunctionTable = nullptr;
+    #endif
+    GPAFuncTableInfo* g_pFuncTableInfo = NULL;
 
     bool InitializeGPA()
     {