Fix header underline in install.rst #138

All 647 tests pass in 6m 49s

647 tests ±0 647 ✔️ ±0 6m 49s ⏱️ +18s
    1 suites ±0     0 💤 ±0
    1 files ±0     0 ❌ ±0

Results for commit 361879e. ± Comparison against earlier commit 0e743c1.