docs: requirements.rst -> requirements.txt

docs/extensions.rst

@@ -9,7 +9,7 @@ Finding Extensions
 
 Flask extensions are listed on the `Flask Extension Registry`_ and can be
 downloaded with ``easy_install`` or ``pip``.  If you add a Flask extension
-as dependency to your ``requirements.rst`` or :file:`setup.py` file they are
+as dependency to your :file:`requirements.txt` or :file:`setup.py` file they are
 usually installed with a simple command or when your application installs.
 
 Using Extensions