doc/go1.5.html: fix typo

Change-Id: If33ffe1da23a8463fc5479f453422d280b736372
Reviewed-on: https://go-review.googlesource.com/11865Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.5.html

@@ -249,7 +249,7 @@ cmd/trace: new command to view traces (https://golang.org/cl/3601)
 The assembler in Go 1.5 is a single new Go program that replaces
 the suite of C-language assemblers (<code>6a</code>,
 <code>8a</code>, etc.) in previous releases.
-The values of the environmetn variables
+The values of the environment variables
 <code>GOARCH</code> and <code>GOOS</code>
 choose which architecture and operating system the generated
 code will be for.