Fix name of tutorial_server_variable.c (#1929)

* Fix name of tutorial_server_variable.c

The example called server_variable.c was removed in https://github.com/open62541/open62541/commit/68402c5ff6e6cfcd1f58b5922dc1df7d865ac697#diff-beb1222a9c51e2083f45b086a9275aa3 .

* Update building.rst

doc/building.rst

@@ -11,8 +11,8 @@ Using the GCC compiler, the following calls build the examples on Linux.
 .. code-block:: bash
 
    cp /path-to/open62541.* . # copy single-file distribution to the local directory
-   cp /path-to/examples/server_variable.c . # copy the example server
-   gcc -std=c99 -DUA_ARCHITECTURE_POSIX open62541.c server_variable.c -o server
+   cp /path-to/examples/tutorial_server_variable.c . # copy the example server
+   gcc -std=c99 -DUA_ARCHITECTURE_POSIX open62541.c tutorial_server_variable.c -o server
 
 Building the Library
 --------------------