tweaks: condense the description of how to report a bug

The list of bugs is quite short these days, and most of them are
feature requests, so asking the user to check whether a particular
problem has already been reported doesn't make much sense.

README

@@ -69,10 +69,8 @@ Mailing Lists
 
 Bug Reports
 
-    To report a bug, please file a description of the problem on nano's
-    bug tracker (https://savannah.gnu.org/bugs/?group=nano -- hover on
-    "Bugs", then click "Submit new").  The issue may have already been
-    reported, so please look first.
+    If you find a bug, please file a detailed description of the problem
+    on nano's bug tracker: https://savannah.gnu.org/bugs/?group=nano.
 
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