Proofread 6-to-7 change summary

[ccjmne]

Summary of fixes:

# Extraneous "the"
-and the the return type
+and the return type

# "little-used" is the proper adjective
-a little used feature
+a little-used feature

# "into" is the proper preposition
-convert any `Observable` in to a `Promise`
+convert any `Observable` into a `Promise`

# Operators use the code highlight everywhere else
-Calling takeLast without
+Calling `takeLast` without

# "in line with" is the proper locution (in two places)
-This puts us in-line with
+This puts us in line with

# Past participle expected here
-handling is use
+handling is used
# ... and there
-information was send
+information was sent

# Typo: "to" instead of "two"
-the equality of to observables
+the equality of two observables