Update C++ style guide

One small update to clarify that omitting braces on control blocks is
permitted but discouraged.

cppguide.html

diff --git a/cppguide.html b/cppguide.html
index c5c7b98..9e849cc 100644
--- a/cppguide.html
+++ b/cppguide.html
@@ -5557,12 +5557,12 @@
   DoSomething();
 </pre>
 
-<p>For historical reasons, we allow one exception to the above rules: the curly
-braces for the controlled statement or the line breaks inside the curly braces
-may be omitted if as a result the entire statement appears on either a single
-line (in which case there is a space between the closing parenthesis and the
-controlled statement) or on two lines (in which case there is a line break
-after the closing parenthesis and there are no braces).</p>
+<p>For historical reasons, we allow, but discourage, one exception to the above
+rules: the curly braces for the controlled statement or the line breaks inside
+the curly braces may be omitted if as a result the entire statement appears on
+either a single line (in which case there is a space between the closing
+parenthesis and the controlled statement) or on two lines (in which case there
+is a line break after the closing parenthesis and there are no braces).</p>
 
 <pre class="neutralcode">// OK - fits on one line.
 if (x == kFoo) { return new Foo(); }