Fix sed command to use FIPS compatible operator image in FIPS doc

[thbkrkr]

This corrects the sed command used to change the container image name to use the FIPS-enabled version in the k8s manifests to deploy the operator.

Reported in 3043fbf by @emrcbrn.

Previous command was renaming the image to:

curl -s https://download.elastic.co/downloads/eck/2.6.1/operator.yaml | sed -r 's#(image:.*eck-operator.*)"#\1-fips"#' | grep fips
image: "docker.elastic.co/eck/eck-operator:2.6.1-fips"

Which isn't found:

docker pull docker.elastic.co/eck/eck-operator:2.6.1-fips
Error response from daemon: manifest for docker.elastic.co/eck/eck-operator:2.6.1-fips not found: manifest unknown: manifest unknown

The new sed command renames it correctly:

curl -s https://download.elastic.co/downloads/eck/2.6.1/operator.yaml | sed -r 's#(image:.*eck-operator)(:.*)#\1-fips\2#' | grep fips
image: "docker.elastic.co/eck/eck-operator-fips:2.6.1"

Which pulls correctly:

docker pull docker.elastic.co/eck/eck-operator-fips:2.6.1
2.6.1: Pulling from eck/eck-operator-fips
fc251a6e7981: Pull complete 
029c2b2b0b4e: Pull complete 
67be15e29e15: Pull complete 
cf5b1a76c1a2: Pull complete 
Digest: sha256:49ba2ed24db1bbb4806709226ec23262bf6adc8db63d6f87751869a4839efc1d
Status: Downloaded newer image for docker.elastic.co/eck/eck-operator-fips:2.6.1
docker.elastic.co/eck/eck-operator-fips:2.6.1