Merge pull request #770 from aashishc1988/ticker_doc

minor changes to ticker doc

Author: Amanda Butler

docs/api/drivers/Ticker.md

@@ -12,7 +12,7 @@ You can create any number of Ticker objects, allowing multiple outstanding inter
 
 - No printf, malloc or new in ISR: avoid any call to bulky library functions. In particular, certain library functions (such as printf, malloc and new) are not re-entrant, and their behavior could be corrupted when called from an ISR.
 
-- While an event is attached to a Ticker, deep sleep is blocked to maintain accurate timing. If you don't need microsecond precision, consider using the LowPowerTicker class instead because this does not block deep sleep mode.
+- While an event is attached to a Ticker, deep sleep is blocked to maintain accurate timing. If you don't need microsecond precision, consider using the LowPowerTicker class instead because that does not block deep sleep mode.
 
 ### Ticker class reference