DOC: shared-libraries: fix and clarify install location example

Fixes #769

Author: AndrewAnnex

docs/how-to-guides/shared-libraries.rst

@@ -64,15 +64,16 @@ A shared library produced by ``library()`` or ``shared_library()`` built like th
 
 is installed to ``libdir`` by default. If the only reason the shared library exists
 is to be used inside the Python package being built, then it is best to modify
-the install location, via the `install_dir` argument, to be within the Python package itself:
+the install location, via the ``install_dir`` argument, to be within 
+the Python package itself:
 
 .. code-block:: meson
 
     example_lib = shared_library(
         'example',
         'examplelib.c',
         install: true,
-        install_dir: py.get_install_dir() / 'mypkg/subdir',
+        install_dir: py3.get_install_dir() / 'mypkg/subdir',
     )
 
 Then an extension module in the same install directory can link against the