os: fix typo in doc

Change-Id: I9797b44dfa7c2c853b7a656f4f722df2c862824b
Reviewed-on: https://go-review.googlesource.com/4991Reviewed-by: Andrew Gerrand <adg@golang.org>

src/os/env.go

@@ -48,7 +48,7 @@ func isAlphaNum(c uint8) bool {
 	return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
 }
 
-// getName returns the name that begins the string and the number of bytes
+// getShellName returns the name that begins the string and the number of bytes
 // consumed to extract it.  If the name is enclosed in {}, it's part of a ${}
 // expansion and two more bytes are needed than the length of the name.
 func getShellName(s string) (string, int) {