Clarify that "or it is dropped" is meant restrictively

The introduction of the *Destructors* chapter says:

> When an initialized variable or temporary goes out of scope,
> its *destructor* is run, or it is *dropped*.

Grammatically, the final comma there suggests that the clause "or it
is dropped" is non-restrictive -- i.e., that it could be removed from
the sentence without changing its meaning.  It's as though we had
written:

> When an initialized variable or temporary goes out of scope,
> its *destructor* is run, or in other words, it is *dropped*.

That's not what we mean here currently, as discussed in [#1078], so
the comma is incorrectly placed.

Note that this isn't necessarily the final step in clarifying this.
There's more we should probably do.  But it is a step in the right
direction, so let's take it.

[#1078]: #1078

Author: traviscross

src/destructors.md

@@ -3,7 +3,7 @@ r[destructors]
 
 r[destructors.intro]
 When an [initialized] [variable] or [temporary] goes out of
-[scope](#drop-scopes), its *destructor* is run, or it is *dropped*. [Assignment]
+[scope](#drop-scopes), its *destructor* is run or it is *dropped*. [Assignment]
 also runs the destructor of its left-hand operand, if it's initialized. If a
 variable has been partially initialized, only its initialized fields are
 dropped.