Correct link to Arduino Board Manager info (#5650)

The link was pointing to Arduino Libraries Manager info and not to Arduino Board Manager info.

Author: lrmoreno007

Diff for: 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
 -----------------