Fix parsing of companion reference line in PR description

[joao-paulo-parity]

Problem: The pull request template of Substrate (example: paritytech/substrate#11392) has the expression "Polkadot companion:" which incorrectly gets interpreted as a companion reference

pipeline-scripts/check_dependent_project.sh

Line 384 in d1d27b3

if [[ "$line" =~ [cC]ompanion:[[:space:]]*([^[:space:]]+) ]]; then

Solution: improve the regular expression to not misdetect such cases. Perhaps ^[^[:space:]]+[[:space:]]+[cC]ompanion:[[:space:]]*([^[:space:]]+)