docs,stm32: Fix minor typos in RTC docs, and->an.

docs/library/machine.RTC.rst

@@ -4,7 +4,7 @@
 class RTC -- real time clock
 ============================
 
-The RTC is and independent clock that keeps track of the date
+The RTC is an independent clock that keeps track of the date
 and time.
 
 Example usage::

docs/library/pyb.RTC.rst

@@ -4,7 +4,7 @@
 class RTC -- real time clock
 ============================
 
-The RTC is and independent clock that keeps track of the date
+The RTC is an independent clock that keeps track of the date
 and time.
 
 Example usage::

ports/stm32/rtc.c

@@ -43,7 +43,7 @@
 /// \moduleref pyb
 /// \class RTC - real time clock
 ///
-/// The RTC is and independent clock that keeps track of the date
+/// The RTC is an independent clock that keeps track of the date
 /// and time.
 ///
 /// Example usage: