doc: Fixed missing dot in effective_go.html

First sentence of https://golang.org/doc/effective_go.html#maps dot is missing.

Change-Id: Ic93b407945a6ad24a73ae9f7b6076ac9619871db
Reviewed-on: https://go-review.googlesource.com/61020Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

doc/effective_go.html

@@ -1521,7 +1521,7 @@ for i := range picture {
 <p>
 Maps are a convenient and powerful built-in data structure that associate
 values of one type (the <em>key</em>) with values of another type
-(the <em>element</em> or <em>value</em>)
+(the <em>element</em> or <em>value</em>).
 The key can be of any type for which the equality operator is defined,
 such as integers,
 floating point and complex numbers,