doc: adjust indentation in Effective Go commentary example.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7225080

doc/effective_go.html

@@ -167,10 +167,9 @@ should set up the detailed documentation that follows.
 
 <pre>
 /*
-    Package regexp implements a simple library for
-    regular expressions.
+Package regexp implements a simple library for regular expressions.
 
-    The syntax of the regular expressions accepted is:
+The syntax of the regular expressions accepted is:
 
     regexp:
         concatenation { '|' concatenation }