all: update 'go get' command in standard library README.vendor

The -m flag is removed in Go 1.13. -d should be used instead.

Change-Id: Ia53764748309f16cb231e5ac6770400a73804484
Reviewed-on: https://go-review.googlesource.com/c/go/+/191621
Run-TryBot: Jay Conrod <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: Jay Conrod

src/README.vendor

@@ -41,13 +41,13 @@ The vendor directory may be updated with 'go mod vendor'.
 A typical sequence might be:
 
     cd src
-    go get -m golang.org/x/net@latest
+    go get -d golang.org/x/net@latest
     go mod tidy
     go mod vendor
 
 Use caution when passing '-u' to 'go get'. The '-u' flag updates
-modules providing all transitively imported packages, not just
-the target module.
+modules providing all transitively imported packages, not only
+the module providing the target package.
 
 Note that 'go mod vendor' only copies packages that are transitively
 imported by packages in the current module. If a new package is needed,