Fix documentation for building the docs

BUILD.md mentioned requirements-build.txt only req file was called
requirements-dev.txt

Author: rasjani

BUILD.md

@@ -27,9 +27,9 @@ tool with a help by our [rellu](https://github.com/robotframework/rellu)
 utilities, but also other tools and modules are needed. A pre-condition
 is installing all these, and that\'s easiest done using
 [pip](http://pip-installer.org) and the provided
-[requirements-build.txt](requirements-build.txt) file:
+[requirements-dev.txt](requirements-dev.txt) file:
 
-    pip install -r requirements-build.txt
+    pip install -r requirements-dev.txt
 
 ## Using Invoke