docs: Change variable in pkg-config example

The example provided for the `get_pkgconfig_variable` when using using the `define_variable` parameter is not the best example, because it is using `prefix` for both. This changes the retrieved variable for `libdir` so the efect of the variable redefinition is more noticeable.

docs/markdown/Dependencies.md

@@ -55,7 +55,7 @@ These variables can also be redefined by passing the `define_variable`
 parameter, which might be useful in certain situations:
 
 ```meson
-zdep_prefix = zdep.get_pkgconfig_variable('prefix', define_variable: ['prefix', '/tmp'])
+zdep_prefix = zdep.get_pkgconfig_variable('libdir', define_variable: ['prefix', '/tmp'])
 ```
 
 The dependency detector works with all libraries that provide a