correction in variable declartion (#5160)

`int i;` is a definition of the object, since it will allocate storage.

Author: mskhan001

docs/cpp/program-and-linkage-cpp.md

@@ -11,7 +11,7 @@ In a C++ program, a *symbol*, for example a variable or function name, can be de
 The following example shows some declarations:
 
 ```cpp
-int i;
+extern int i;
 int f(int x);
 class C;
 ```