[readme] use single quotes in nested ternary bad example (section 15.6)

The bad example in section 15.6 (nested ternaries) used double-quoted
string literals ('"bar"', '"baz"'), which contradicts section 6.1 of
this same style guide that requires single quotes.

Also related to issue #3152 which flagged inconsistent style conventions
in code examples across sections.

Fixes the bad example to use single-quoted strings consistently with
the rest of the guide and with the good examples immediately below.

Author: eduardbar

README.md

@@ -2133,8 +2133,8 @@ Other Style Guides
     ```javascript
     // bad
     const foo = maybe1 > maybe2
-      ? "bar"
-      : value1 > value2 ? "baz" : null;
+      ? 'bar'
+      : value1 > value2 ? 'baz' : null;
 
     // split into 2 separated ternary expressions
     const maybeNull = value1 > value2 ? 'baz' : null;