style-guide: Fix chain example to match rustfmt behavior

The style guide gave an example of breaking a multi-line chain element
and all subsequent elements to a new line, but that same example and the
accompanying text also had several chain items stacked on the first
line. rustfmt doesn't do this, except when the rule saying to combine

```
shrt
    .y()
```

into

```
shrt.y()
```

applies.

Author: joshtriplett

src/doc/style-guide/src/expressions.md

@@ -450,12 +450,11 @@ foo(
 
 #### Multi-line elements
 
-If any element in a chain is formatted across multiple lines, then that element
-and any later elements must be on their own line. Earlier elements may be kept
-on a single line. E.g.,
+If any element in a chain is formatted across multiple lines, put that element
+and any later elements on their own lines.
 
 ```rust
-a.b.c()?.d
+a.b.c()?
     .foo(
         an_expr,
         another_expr,