Minor fixes (#613)

* Minor doc syntax fix

The numpy documentation had a bad :file: reference (was using double
backticks instead of single backticks).

* Changed long-outdated "example" -> "tests" wording

The ConstructorStats internal docs still had "from example import", and
the main testing cpp file still used "example" in the module
description.

tests/pybind11_tests.cpp

@@ -32,7 +32,7 @@ void bind_ConstructorStats(py::module &m) {
 }
 
 PYBIND11_PLUGIN(pybind11_tests) {
-    py::module m("pybind11_tests", "pybind example plugin");
+    py::module m("pybind11_tests", "pybind testing plugin");
 
     bind_ConstructorStats(m);