docs: fix broken link in HTTP header doc (#542)
docs: fix broken link in HTTP header doc (#542)

Signed-off-by: Stephan Zuercher <f965a4a1b587c4e3c1892642aed563e43902502b@turbinelabs.io>
lyft/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,lyft/envoy-api,lyft/envoy-api