Fix quotes in multi-line string syntax (#2638)

This was pointed out by i-khadra on #2132

docs/design/README.md

@@ -606,18 +606,18 @@ String literals may be written on a single line using a double quotation mark
 (`"`) at the beginning and end of the string, as in `"example"`.
 
 Multi-line string literals, called _block string literals_, begin and end with
-three double quotation marks (`"""`), and may have a file type indicator after
-the first `"""`.
+three single quotation marks (`'''`), and may have a file type indicator after
+the first `'''`.
 
 ```carbon
 // Block string literal:
-var block: String = """
+var block: String = '''
     The winds grow high; so do your stomachs, lords.
     How irksome is this music to my heart!
     When such strings jar, what hope of harmony?
     I pray, my lords, let me compound this strife.
         -- History of Henry VI, Part II, Act II, Scene 1, W. Shakespeare
-    """;
+    ''';
 ```
 
 The indentation of a block string literal's terminating line is removed from all