[v1.5.x] Update _index.md (#4770) (#4778)

Add 'download/' to the OPERATOR_SDK_DL_URL

Signed-off-by: Brad P. Crochet <[email protected]>

Author: Eric Stroczynski

website/content/en/docs/installation/_index.md

@@ -36,7 +36,7 @@ export OS=$(uname | awk '{print tolower($0)}')
 Download the binary for your platform:
 
 ```sh
-export OPERATOR_SDK_DL_URL=https://github.com/operator-framework/operator-sdk/releases/v1.5.1
+export OPERATOR_SDK_DL_URL=https://github.com/operator-framework/operator-sdk/releases/download/v1.5.1
 curl -LO ${OPERATOR_SDK_DL_URL}/operator-sdk_${OS}_${ARCH}
 ```