Change double quotes to single quotes

- "." is a string literal, and so is treated as the char '.' plus the null char '\0'.
- Single quotes reduces the necessary memory for this literal to only one char instead.
  A string literal as the one actually present may require the use of the
  method "write(const char *str)", so there could be also a performance overhead.
- Another reason to change quotes style is for consistency with line 235.
Signed-off-by: Patrick Roncagliolo <ronca.pat@gmail.com>

cores/arduino/Print.cpp

@@ -250,7 +250,7 @@ size_t Print::printFloat(double number, uint8_t digits)
 
   // Print the decimal point, but only if there are digits beyond
   if (digits > 0) {
-    n += print("."); 
+    n += print('.'); 
   }
 
   // Extract digits from the remainder one at a time