Format markdown (knative#266)

Produced via:
  `prettier --write --prose-wrap=always $(find -name '*.md' | grep -v vendor | grep -v .github)`
/assign k4leung4
/cc k4leung4

README.md

@@ -186,4 +186,5 @@ EOF
 
 ## Upgrade
 
-Please refer to the [upgrade guide](doc/upgrade_guide.md) for a safe upgrade process.
+Please refer to the [upgrade guide](doc/upgrade_guide.md) for a safe upgrade
+process.

doc/upgrade_guide.md

@@ -1,27 +1,32 @@
 # Upgrade Guide
 
-This document describes how to upgrade the serving-operator to an expected version.
+This document describes how to upgrade the serving-operator to an expected
+version.
 
 ## Backup precaution
 
-As an administrator, you are recommended to save the content of the custom resource for serving-operator
-before upgrading your operator. Make sure that you know the name and the namespace of your CR, and use the
-following command to save the CR in a file called `serving_operator_cr.yaml`:
+As an administrator, you are recommended to save the content of the custom
+resource for serving-operator before upgrading your operator. Make sure that you
+know the name and the namespace of your CR, and use the following command to
+save the CR in a file called `serving_operator_cr.yaml`:
 
 For the version v0.10.0 or later:
 
 ```
 kubectl get KnativeServing <cr-name> -n <namespace> -o=yaml > serving_operator_cr.yaml
 ```
 
-Replace `<cr-name>` with the name of your CR, and `<namespace>` with the namespace.
+Replace `<cr-name>` with the name of your CR, and `<namespace>` with the
+namespace.
 
-One version of serving-operator installs only one specific version of Knative Serving. With your
-operator successfully upgraded, your Knative Serving is upgraded as well.
+One version of serving-operator installs only one specific version of Knative
+Serving. With your operator successfully upgraded, your Knative Serving is
+upgraded as well.
 
 ## v0.10.0, v0.11.x -> v0.12.0
 
-Both of v0.10 and v0.11 versions are able to upgrade to the version v0.12.0 by running the following command:
+Both of v0.10 and v0.11 versions are able to upgrade to the version v0.12.0 by
+running the following command:
 
 ```
 kubectl apply -f https://github.com/knative/serving-operator/releases/download/v0.12.0/serving-operator.yaml