Correct link to Arduino Board Manager info (#5650)

lrmoreno007 · d-a-v · commit 39e31b23529e · 2019-01-22T17:29:44.000+01:00
The link was pointing to Arduino Libraries Manager info and not to Arduino Board Manager info.
diff --git a/doc/installing.rst b/doc/installing.rst
@@ -35,7 +35,7 @@ might be broken.
 
 For more information on the Arduino Board Manager, see:
 
-- https://www.arduino.cc/en/Guide/Libraries
+- https://www.arduino.cc/en/guide/cores
 
 Using git version
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