DOC: replaced 'Good as first PR' with 'good first issue' (pandas-dev#18806)

Author: Ray Bell

doc/README.rst

@@ -160,8 +160,8 @@ Where to start?
 
 There are a number of issues listed under `Docs
 <https://github.com/pandas-dev/pandas/issues?labels=Docs&sort=updated&state=open>`_
-and `Good as first PR
-<https://github.com/pandas-dev/pandas/issues?labels=Good+as+first+PR&sort=updated&state=open>`_
+and `good first issue
+<https://github.com/pandas-dev/pandas/issues?labels=good+first+issue&sort=updated&state=open>`_
 where you could start out.
 
 Or maybe you have an idea of your own, by using pandas, looking for something