doc: remove meaningless word from Go 1.12 release notes

Change-Id: I744940e2bbde19ccec53af6c5469d46ba9161f01
Reviewed-on: https://go-review.googlesource.com/c/160179
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/go1.12.html

@@ -301,7 +301,7 @@ <h3 id="godoc"><code>godoc</code> and <code>go</code> <code>doc</code></h3>
 
 <p><!-- CL 140959 -->
  <code>go</code> <code>doc</code> also now includes the <code>-src</code> flag,
- which will show the target's original source code.
+ which will show the target's source code.
 </p>
 
 <h3 id="trace">Trace</h3>