doc: improve clarity of map index examples

The fourth example for map indexing states you have a map of type [K]V
and attempts to read in a variable of type T.  Further, the example
is meant to showcase the boolean return variable saying whether the
map contained a key, but overrides to type T.  This will not compile.

Changed last updated date to February 18

Fixes: #23895

Change-Id: I63c52adbcd989afd4855e329e6c727f4c01f7881
Reviewed-on: https://go-review.googlesource.com/94906Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of February 1, 2018",
+	"Subtitle": "Version of February 18, 2018",
 	"Path": "/ref/spec"
 }-->
 
@@ -3051,7 +3051,6 @@ used in an <a href="#Assignments">assignment</a> or initialization of the specia
 v, ok = a[x]
 v, ok := a[x]
 var v, ok = a[x]
-var v, ok T = a[x]
 </pre>
 
 <p>