Use correct function name in readStringUntil() syntax section

Fixes regression of previously reported/fixed issue arduino/Arduino#3734

Author: per1234

Language/Functions/Communication/Serial/readStringUntil.adoc

@@ -24,7 +24,7 @@ This function is part of the Stream class, and is called by any class that inher
 
 [float]
 === Syntax
-`Serial.readString(terminator)`
+`Serial.readStringUntil(terminator)`
 
 
 [float]