Fix env variable name typo (opensearch-project#14843)

Signed-off-by: Rishabh Singh <sngri@amazon.com>
Signed-off-by: Kaushal Kumar <ravi.kaushal97@gmail.com>

Author: rishabh6788

.github/workflows/benchmark-pull-request.yml

@@ -115,7 +115,7 @@ jobs:
           headRepo=$(echo '${{ steps.get_pr.outputs.result }}' | jq -r '.headRepoFullName')
           headRef=$(echo '${{ steps.get_pr.outputs.result }}' | jq -r '.headRef')
           echo "prHeadRepo=$headRepo" >> $GITHUB_ENV
-          echo "prheadRef=$headRef" >> $GITHUB_ENV
+          echo "prHeadRef=$headRef" >> $GITHUB_ENV
       - name: Checkout PR Repo
         uses: actions/checkout@v2
         with: