Updated creation script and README to match generation script

tests/syntax-tests/source/Plaintext/README.md

@@ -1,10 +1,12 @@
-This text file was generated with the following script. Certain invalid characters were removed manually:
+This text file was generated with the following script.
 
 ```python
 with open("plaintext.txt", "w"):
-    for i in range(0x10FFFF):
+    for i in range(176):
         try:
             f.write(chr(i) + "\n")
         except:
             pass
+    f.write("\n")
+    f.write("Here is a line with multiple characters")
 ```