Fixes an invalid documentation for the .gitlab-ci.yml cache:key

doc/ci/yaml/README.md

@@ -276,7 +276,7 @@ To enable per-branch caching:
 ```yaml
 cache:
-  key: "$CI_COMMIT_REF_NAME"
+  key: "$CI_COMMIT_REF_SLUG"
   untracked: true
 ```
@@ -284,7 +284,7 @@ To enable per-job and per-branch caching:
 ```yaml
 cache:
-  key: "$CI_JOB_NAME-$CI_COMMIT_REF_NAME"
+  key: "$CI_JOB_NAME-$CI_COMMIT_REF_SLUG"
   untracked: true
 ```
@@ -292,7 +292,7 @@ To enable per-branch and per-stage caching:
 ```yaml
 cache:
-  key: "$CI_JOB_STAGE-$CI_COMMIT_REF_NAME"
+  key: "$CI_JOB_STAGE-$CI_COMMIT_REF_SLUG"
   untracked: true
 ```
@@ -301,7 +301,7 @@ If you use **Windows Batch** to run your shell scripts you need to replace
 ```yaml
 cache:
-  key: "%CI_JOB_STAGE%-%CI_COMMIT_REF_NAME%"
+  key: "%CI_JOB_STAGE%-%CI_COMMIT_REF_SLUG%"
   untracked: true
 ```
@@ -310,7 +310,7 @@ If you use **Windows PowerShell** to run your shell scripts you need to replace
 ```yaml
 cache:
-  key: "$env:CI_JOB_STAGE-$env:CI_COMMIT_REF_NAME"
+  key: "$env:CI_JOB_STAGE-$env:CI_COMMIT_REF_SLUG"
   untracked: true
 ```