fix(scripts): backticks in a double-quoted echo run as command substitution

Line 19 wrapped an example in backticks inside a double-quoted string. Backticks there are COMMAND SUBSTITUTION, so bash tried to execute '+ uses: …@<sha>' and printed the message with the example silently deleted. Switched to single quotes. Same defect class as hyperpolymath/Axiom.jl#82.

Found by an estate-wide shellcheck sweep of 5,111 tracked scripts across 375
repos. This file was one of 11 that fail to PARSE (SC1073/SC1072) — shellcheck
stops analysing at the failure, so anything after it was never checked either.

Verified: shellcheck -S error now reports 0 findings for this file.

Author: hyperpolymath

scripts/fix-close-obsolete-pr.sh

@@ -16,7 +16,7 @@ REPO_NWO=$(echo "$URL" | awk -F'/' '{print $4"/"$5}')
 DIFF=$(gh pr diff "$URL" 2>/dev/null)
 PR_USES=$(echo "$DIFF" | grep -E "^\+[[:space:]]+uses:[[:space:]]+[^@[:space:]]+@[0-9a-f]{40}" || true)
 if [ -z "$PR_USES" ]; then
-    echo "SKIP: PR has no `+ uses: …@<sha>` lines"
+    echo 'SKIP: PR has no `+ uses: …@<sha>` lines'
     exit 0
 fi