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/94906


Reviewed-by: Robert Griesemer <gri@golang.org>