PEP 586: Fix typos and tighten wording (#929)

This pull request fixes one or two typos in PEP 586.

It also slightly adjusts the wording of the section on
type inference as suggested here:
Michael0x2a#1 (comment)

pep-0586.rst

@@ -58,7 +58,7 @@ Note that this problem persists even if we redesign these APIs to
 instead accept enums: ``MyEnum.FOO`` and ``MyEnum.BAR`` are both
 considered to be of type ``MyEnum``.
 
-Currently, type checkers work around this limitation by adding ad-hoc
+Currently, type checkers work around this limitation by adding ad hoc
 extensions for important builtins and standard library functions. For
 example mypy comes bundled with a plugin that attempts to infer more
 precise types for ``open(...)``. While this approach works for standard
@@ -379,7 +379,7 @@ special-casing. For example, programs like the following are type safe::
    expects_str(var)
 
 This also means non-Literal expressions in general should not automatically
-inferred to be Literal. For example::
+be cast to Literal. For example::
 
    def expects_literal(x: Literal["foo"]) -> None: ...
 
@@ -623,7 +623,7 @@ It's entirely possible we'll circle back and revisit this topic in the future:
 we very likely will need some form of dependent typing along with other
 extensions like variadic generics to support popular libraries like numpy.
 
-This PEP should be seen as a stepping stones towards this goal,
+This PEP should be seen as a stepping stone towards this goal,
 rather then an attempt at providing a comprehensive solution.
 
 Adding more concise syntax