Add suggestions from PR #67 review

Author: sebromero

libraries/Portenta_System/examples/PortentaH7_getBootloaderInfo/PortentaH7_getBootloaderInfo.ino

@@ -30,11 +30,11 @@ String getUSBSpeed(uint8_t flag) {
 String getClockSource(uint8_t flag) {
   switch (flag){
   case 0x8:
-    return "External clock (ST Link MCO)";
+    return "External oscillator";
   case 0x4:
-    return "External xtal (X3 on board - not provided by default)";
+    return "External crystal";
   case 0x2: 
-    return "HSI internal clock"; 
+    return "Internal clock"; 
   default:
     return "N/A";
   }