fix cmake dir

README.rst

@@ -191,7 +191,7 @@ equal to :code:`<TURTLE_DIR>`.
         make install
 
     The directory `<CMAKE_DIR>` is only relevant to later executing
-    the `cmake` binary (which can be found under `${PREFIX}/bin` after
+    the `cmake` binary (which can be found under :code:`<CMAKE_DIR>/bin` after
     installation).