Clean up comments in platform.txt

- Remove comment in Arduino AVR Boards platform.txt about -w flag. This
is no longer relevant since the compiler warning level is now set by the
user.
- Fix typos

Author: per1234

platform.txt

@@ -36,7 +36,7 @@ compiler.ldflags=
 compiler.size.cmd=arm-none-eabi-size
 compiler.define=-DARDUINO=
 
-# this can be overriden in boards.txt
+# This can be overridden in boards.txt
 build.extra_flags=
 
 # These can be overridden in platform.local.txt