doc: fix minor capitalization typo

Updated page to Page to match with the sample code

Fixes #21773

Change-Id: Ia884a22fd587860c7a6148103b2b474425e45284
Reviewed-on: https://go-review.googlesource.com/66790Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/articles/wiki/index.html

@@ -179,7 +179,7 @@ You can compile and run the program like this:
 <pre>
 $ go build wiki.go
 $ ./wiki
-This is a sample page.
+This is a sample Page.
 </pre>
 
 <p>