Fix an RST link typo in the dev guide

docs/developer-guide.txt

@@ -139,7 +139,7 @@ Vendored Dependencies
 ---------------------
 
 Setuptools has some dependencies, but due to `bootstrapping issues
-<https://github.com/pypa/setuptools/issues/980>`, those dependencies
+<https://github.com/pypa/setuptools/issues/980>`_, those dependencies
 cannot be declared as they won't be resolved soon enough to build
 setuptools from source. Eventually, this limitation may be lifted as
 PEP 517/518 reach ubiquitous adoption, but for now, Setuptools