Update comments to reflect Stream functions changed from private to protected

These functions were changed from private to protected in https://github.com/arduino/Arduino/commit/99f2a2755349784835130147e46cb61659b85893 but the comments were not updated at that time.

In conjunction with equivalent pull requests to Arduino SAM Boards and Arduino SAMD Boards, solves https://github.com/arduino/Arduino/issues/6146.

cores/arduino/Stream.cpp

@@ -27,7 +27,7 @@
 
 #define PARSE_TIMEOUT 1000  // default number of milli-seconds to wait
 
-// private method to read stream with timeout
+// protected method to read stream with timeout
 int Stream::timedRead()
 {
   int c;
@@ -39,7 +39,7 @@ int Stream::timedRead()
   return -1;     // -1 indicates timeout
 }
 
-// private method to peek stream with timeout
+// protected method to peek stream with timeout
 int Stream::timedPeek()
 {
   int c;

cores/arduino/Stream.h

@@ -51,8 +51,8 @@ class Stream : public Print
   protected:
     unsigned long _timeout;      // number of milliseconds to wait for the next char before aborting timed read
     unsigned long _startMillis;  // used for timeout measurement
-    int timedRead();    // private method to read stream with timeout
-    int timedPeek();    // private method to peek stream with timeout
+    int timedRead();    // read stream with timeout
+    int timedPeek();    // peek stream with timeout
     int peekNextDigit(LookaheadMode lookahead, bool detectDecimal); // returns the next numeric digit in the stream or -1 if timeout
 
   public: