Web installer: correctly show note for I4ecd0659

InstallDocFormatter didn't escape the double underscores in
__TOC__. I also noticed that wfEscapeWikiText had a similar
bug, which I reported as bug 53658 (fixed separately).

Change-Id: Ib5346b9c234f1c5ef361cc4357352b09def5dc25
Merge "Web installer: correctly show note for I4ecd0659"