[docs] Fixing typo in the doc (#452)

* Fixing typo for web frameworks

Author: alxch-

docs/index.rst

@@ -73,7 +73,10 @@ Instrumentation
 Web
 ~~~
 
-We support many :ref:`web-frameworks`. Install the middleware for yours.
+We support many `web frameworks`_. Install the middleware for yours.
+
+.. _web frameworks: #web-frameworks
+
 
 Databases
 ~~~~~~~~~