Fix missing word in comment (#1178)

Line 22 (https://github.com/linkerd/linkerd2-proxy/blob/cde2816a85e2e31ce5e95ba15f41934a178b9a96/linkerd/proxy/api-resolve/src/metadata.rs#L29) of linkerd/api-resolve/src/metadata.rs has a grammar issue, where the word "know" is missing.

Author: iojcde

linkerd/proxy/api-resolve/src/metadata.rs

@@ -26,7 +26,7 @@ pub struct Metadata {
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub enum ProtocolHint {
-    /// We don't what the destination understands, so forward messages in the
+    /// We don't know what the destination understands, so forward messages in the
     /// protocol we received them in.
     Unknown,
     /// The destination can receive HTTP2 messages.