doc: clarify a sentence about *_js.go

Change "have to" to "need to" for clarity and to avoid a
peculiar English idiom.

Change-Id: Iec2b1f841d0353dd7925f8f934fe82d4ed059d7d
Reviewed-on: https://go-review.googlesource.com/123495Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.11.html

@@ -63,7 +63,7 @@ Do not send CLs removing the interior tags from such phrases.
   Go files named <code>*_js.go</code> or <code>*_wasm.go</code> will
   now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored by Go
   tools</a> except when those GOOS/GOARCH values are being used.
-  If you have existing filenames matching those patterns, you will have to rename them.
+  If you have existing filenames matching those patterns, you will need to rename them.
 </p>
 
 <h2 id="tools">Tools</h2>