doc/install-source.html: fix cd command, already in go dir.

Change-Id: I42929abe5cefd84a1daf4f36ee1806c490ce16ec
Reviewed-on: https://go-review.googlesource.com/5960Reviewed-by: Minux Ma <minux@golang.org>

doc/install-source.html

@@ -144,7 +144,7 @@ To build the Go distribution, run
 </p>
 
 <pre>
-$ cd go/src
+$ cd src
 $ ./all.bash
 </pre>