Fix type

Co-authored-by: Juuz <[email protected]>

src/main/java/net/fabricmc/mappingio/MappingReader.java

@@ -58,7 +58,7 @@ public static MappingFormat detectFormat(Reader reader) throws IOException {
 		int pos = 0;
 		int len;
 
-		// Be careful not to close the reader, thats up to the caller.
+		// Be careful not to close the reader, that's up to the caller.
 		BufferedReader br = reader instanceof BufferedReader ? (BufferedReader) reader : new BufferedReader(reader);
 
 		br.mark(DETECT_HEADER_LEN);