Small fixes in FAQ.
Small fixes in FAQ.

Fix a capital letter in title.
Reword the immutable variable paragraph to avoid the double 'instead of'.
Better explained HTML chars in the string interpolation error when using
brackets for tags being seen as key access to hash.

dmaestro/doc,dmaestro/doc,perl6/doc,gfldex/doc,MasterDuke17/doc,dmaestro/doc,antquinonez/doc,antquinonez/doc,stmuk/doc,antquinonez/doc,gfldex/doc,tbrowder/doc,jonathanstowe/doc,tbrowder/doc,perl6/doc,jonathanstowe/doc,dmaestro/doc,gfldex/doc,tbrowder/doc,stmuk/doc,stmuk/doc,antquinonez/doc,jonathanstowe/doc,perl6/doc,MasterDuke17/doc,stmuk/doc,dmaestro/doc,perl6/doc,MasterDuke17/doc,antquinonez/doc,perl6/doc,jonathanstowe/doc,gfldex/doc,gfldex/doc,tbrowder/doc,stmuk/doc,MasterDuke17/doc,jonathanstowe/doc