Typo In Subject.asObservable() doc header

[pitoniak32]

rxjs/src/internal/Subject.ts

Line 138 in 081ca2b

* to create customize Observer-side logic of the Subject and conceal it from

it appears there is a small typo in the doc header for the asObservable function in Subject.

recommendation:

/*
* ...
* to create customizable Observer-side logic of the Subject and conceal it from
* ...
*/