Copter: update Release notes again for 2.9.1b release

Missed item re increasing max inav accel correction to 3 m/s

ArduCopter/ReleaseNotes.txt

@@ -4,9 +4,10 @@ ArduCopter 2.9.1b 30-Feb-2013
 Improvements over 2.9.1:
 1) reduce INS_MPU6K_FILTER to 20hz
 2) reduce InertialNav Z-axis time constant to 5 (was 7)
-3) bug fix for alt_hold being passed as int16_t to get_throttle_althold_with_slew
-4) bug fix for throttle after acro flip (was being kept at min throttle if pilot switched out of ACRO mode while inverted)
-5) reduce yaw_rate P default to 0.20 (was 0.25)
+3) increased max InertialNav accel correction to 3 m/s (was 1m/s)
+4) bug fix for alt_hold being passed as int16_t to get_throttle_althold_with_slew
+5) bug fix for throttle after acro flip (was being kept at min throttle if pilot switched out of ACRO mode while inverted)
+6) reduce yaw_rate P default to 0.20 (was 0.25)
 
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 ArduCopter 2.9.1 & 2.9.1-rc2 01-Feb-2013